Rachael Stirling talks Detectorists, her mum Diana Rigg and being a "muso muse" to Elbow's Guy Garvey

"I have never met anybody who likes The Detectorists that I don't like," she tells me as we sit in her PR's office not far from London's Seven Dials, her hometown. "If the sensibility of it appeals to something in somebody's character then I'm going to like them because it's so gentle and un-mean and un-malicious. It's like a vital vitamin to some people."

Stirling plays Crook's partner Becky in Detectorists. It's possible that I'm a little bit in love with Becky. Or maybe it's Stirling herself, all husky of voice and docker in sweariness. After all, I did carry a real torch for her mum once upon a time. It's not, I tell her, that I decided to go to Stirling University back in the 1980s just to meet Diana Rigg. (Rigg had strong associations with the university, living as she did nearby. Some years later she even became chancellor.) But it's possible that the chance I might was part of the equation.

"Did she give you your degree?" Stirling asks. I'm afraid not, I tell her. "Disappointing," she says, laughing uproariously. 

Stirling has her mum's cheekbones, a voice that sounds like nicotine and tobacco making out and a diary that is not so much full as fatigued. "I've had an amazing year," she admits. This autumn we've not only seen her in Detectorists but she's been playing Toby Jones' wife in the recent BBC One London drama Capital (confusingly, as Jones plays Crook's best friend in Detectorists. "I know. I'm a slut. I'm a tart," she says when I point out as much to Stirling. "What can I tell you? Either that or casting directors don't have much imagination.")

She also plays Winston Churchill's daughter in an ITV drama, Churchill's Secret, that will air in the new year. It's a plum pudding of a thing. Stick your finger in and you'll come out with a big actorly name. Among the turns alongside Stirling are Matthew Macfadyen, Tara Fitzgerald and Michael Gambon as the "great" man himself (we can debate historical worth at another juncture).

There's also a Second World War film Their Greatest Hour and a Half in the offing, directed by Lone (An Education) Scherfig, in which she plays an "acid-tongued ginger lesbian". Called Phyllis. "No sex, please," she points out. "It's rated for 12." She possibly needs to be clear on this because Stirling has lesbian history. She first came to prominence in the TV version of Sarah Waters' novel Tipping the Velvet and that was nothing but sex.

Really, though, I just want to talk to her about Detectorists. There is going to be a Christmas special, she says, but it's only going to feature "Mac and Tobes". No Becky then, which is a huge disappointment.

But Rachael, I say, I do have one worry about the programme. Over and above my fear that there won't be a third series, that is. Here's the thing. Does it suggest that decent men, good men like Mackenzie Crook and Toby Jones, are, when it comes down to it, basically just feckless? Does it present a picture of masculinity that is encouragingly anti-macho yet also essentially useless? "Oh my god," she (almost) shouts. "I think it does the opposite."

Really? So tell me, who wears the trousers, you or Mackenzie? "Clearly Becky wears the trousers in that relationship." And who changes the fuses? "Definitely Becky." My point proved, I say.

But she's not having it. "People have said, 'Oh, we don't quite believe in that couple.' I believe in them. I believe in his wonderful peccadilloes and his curiosity. Mackenzie is quite like his character: his curiosity for things, his sense of humour. And he's a very sexy man. I totally believe that a ball-breaker like Becky would go with someone like Mackenzie. He's the yin to her yang."

This seems a good point to talk about men and her attitude to them. She once said that she was attracted to an "old-fashioned masculinity". Is that still the case, I wonder? "I suppose I'm quite manly in lots of ways but I like to be made to feel like a woman."

Manly? I hadn't noticed. "I think I've got a lot of testosterone coursing through my veins. Who knows how to define masculine or feminine any more? But when going out on a date I think there are certain old-fashioned manners that I still enjoy. I don't mean that as an anti-feminist comment. I just mean it as a pro-women comment. There must be a place for us to exist and our differences to exist without one taking away from the other."

These days she's seeing Guy Garvey. That bloke who sings with the band Elbow if you didn't know. They met at a wedding. Has he written a song about her yet?

"Loads," she laughs. You're a muse now, Rachael. "Um, muso muse. Yeah … it's different. He puts his heart on his sleeve and it so happens to be half my heart now. A lot of his solo album Courting the Squall is about us. We've had conversations about it. He's never hidden anything from anybody ever. And I don't really want every single piece of my life to be out there. So we're just negotiating now. But he's amazing. I'm just a northern bird through and through."

Well yes. She might live in London and have spent much of her childhood there but she sees herself as Scottish. While her mum is acting royalty, her dad, Archie Stirling, is a landowner, a businessman and someone who has tinkered more than a little in theatreland. He's also the laird of the Keir Estate, of course.

"In my bones I feel like a Scot. I always have," Stirling says. "My mum's from Doncaster so whatever that is as a combination of Scotland and Yorkshire. It isn't southern."

Stirling spent her youth shuttling between London and her father's home in Scotland. "Where dad lives I know every hole in every field and everything on every tree and I swim in the river from April until October. And the view of Ben Ledi and Ben Vorlich from Stirling …" She can't find the words to describe it.

"Every holiday I would get on an aeroplane or the sleeper train. We had some hilarious journeys on that with the gerbil in the Quality Street tin and the dog in the basket and the drunken guard."

That continued after her parents divorced in 1990 while she was at boarding school. She's bored of talking about their divorce. It happened when she was 12. She's 38 now. It's not relevant. Plus, the facts as reported – the newspapers led on the story that her dad had an affair with Joely Richardson – she says are an "absolute pile of rubbish". All that matters is that both of her parents are still very much part of her life and vice versa.

What about boarding school, Rachael. Was it character building or a cruel and unusual punishment? "Odd. Odd. Children are resilient. I really love the women who I met there and I really love who they've become. But it definitely develops a survival instinct in you early on which I don't know what I'd do without now because it's part of my equipment. But there was a great deal of bullying."

You were bullied? "Oh yeah. I was really homesick when I first arrived. For two years I was sobbing and circling tears on my letters. I was not egregiously bullied and no bodily harm was inflicted but I was just a weak one in the pack because I was desperately homesick."

But eventually she found her feet. I'm presuming, I tell her, to get a voice like hers she spent her teenage years working her way through packets of Benson & Hedges behind the bicycle shed. "No. I was quite a good girl. I didn't misbehave until I was about 17." She did start dodging chapel for fags, she says. "I was naughty by the end but I wasn't rebellious. You've got to be presented with that authority. You've got to find some way of exercising your right to be a teenager."

On the plus side, she says, the school had incredible facilities. "One of them was this arts centre and inside there was a theatre and a rehearsal space and an exhibition space and I remember when I was let loose on it in my second year. I started to put roots down and feel more confident. That was really the turning point of boarding school in the frame."

She hadn't really been interested in acting before that. "I'd been backstage in a theatre but I didn't really know what Mum did. I knew what she was and I knew people would look at her funny. Dads especially."

Yeah, well, let's move on, Rachael.

She admits that when she got her first big job – a part in the film Still Crazy – she didn't really know what she was doing. "I didn't have a f****** clue what I was doing," she says. It took her 12 takes to walk through a door and hit her mark. "By the time I did Billy Connolly was clapping slow and sarcastically. Thanks, dude."

She was still a student at Edinburgh University at the time. During filming she'd fly back up to Scotland for tutorials. Her mum, I've read, was against her acting. "No, she sort of dismissed it, I think, out of a lovely protective quality. And also it's a f****** cliche … I've got to stop swearing like a trouper … I'm a living, breathing, walking, boringly traditional cliche.

"But the thing is it does happen in other professions and I have tried my hardest not to be conceited about it. I've never waved it. I've got a different surname. I very much got my jobs off my own back. If I've had a sniff of nepotism I run off in the opposite direction. In the end you've got to serve the script. If you can't do that you're not going to get the job."

If anything maybe there's a reverse nepotism at play now anyway. After all her mum does turn up in Detectorists playing her mum.

We have to talk about sex now Rachael, I say. Tipping the Velvet. Nudity. Lesbianism. Being painted gold and wearing a strap-on leather dildo and nothing else. It's fair to say that when it aired in 2002, the series got something of a reaction. Does that say something about our attitude to sex perhaps? "Yes, I think it does. It was sensationalised. But of course it was kind of sensational. I think there were places we could have bedded it in reality. It was quite glossy, so therefore it laid itself open to sensationalism because it seemed to be lesbianism airbrushed and therefore it's never going to be taken that seriously.

"So I think they made a product knowing what the reaction was going to be and fully expecting it and soliciting it to a point, whereas I was in the middle of it and didn't quite realise that they were going to say in The Sun, 'Flick over from the football to see my left nipple at 9.21' or whatever it did."

She doesn't regret it though. Well, maybe apart from the accent, which was authentic 1880s Whitstable. "I decided to go all f****** method and as a result I sound like Dick van Dyke."

The gold paint, though, that was fine. Mostly. "I had a large dram and a half of whisky before that. I had the best makeup artist in the world who was literally going, 'Rachael, bend over,' and painting my … "

She doesn't finish that sentence but you get the idea.

After Tipping she was, she admits, "hot" for five minutes and asked to go to parties in borrowed dresses. ("Not my idea of fun.") Maybe, she says, she could have done with a bit of professional advice at that part beyond what her mum could give her. "I was just a bit ditzy about it. I did some really dodgy bottom-feeding films and then I started doing theatre and started finding my feet and feeling a bit more confident in myself."

There have been periods since Tipping the Velvet where she hasn't worked. For a time she even got a job behind a bar pulling pints. But for the moment she's an actor and a busy one.

Well, actually right now she's on a break until after Christmas. It will give her a chance to catch up with friends and family. "I think you've got to breathe and reboot. I think if you went from one job to another … Lots of people do it. Maybe I'm just lazy … But I think if you just go from Winnebago to Winnebago then what is it you're seeing in the great big wide world that you can put into anything new?
"This morning I've been to my ballet class and seen my birds that I do ballet with. On Saturdays I volunteer to teach kids to read. I'm taking my god-daughter to Winter Wonderland. I've got to invest in these friendships and the kind, patient people who don't mind when you disappear into the ether for a few weeks or months at a time."

Oh, and she's getting to watch a bit of TV. "I watch trash. Absolute brainless trash. I can watch The Real Housewives of New York City/LA and fall asleep in front of it. I find it gruesomely fascinating."

Rachael Stirling is busy and happy and maybe, finally, comfortable in her own skin. "I like my thirties and I'm really looking forward to my forties. I think one of my many weaknesses is wanting to please people, wanting to be liked and I think it's quite an infantile mentality. And as I get older I realise you can't please everybody and it doesn't matter. And that's something I'm getting to now."

Meanwhile, I've bought my sister the Detectorists box set for Christmas. But don't tell her.

Detectorists Christmas Special is on BBC Four on December 23 at 10pm. Churchill's Secret airs in the new year.