Interview: David Mundell, Scotland's only Tory MP

But despite his solitary confinement, the former solicitor, who in the mid-1980s was a councillor for David Owen's SDP, seems upbeat, cheerful even. Indeed, he jokes further that there are certain advantages to being the leader of a parliamentary grouping of one.

"The Conservative Group for Scotland means we're always in agreement with ourselves. I'm not tossing and turning every night, saying will I agree with myself or not. The Scottish Conservative Group within the parliamentary party has an agreed position on everything."

And yet the 51-year-old would like some chums. At present, he has to make do with his Tory colleagues south of the border and then, of course, there is the group of 59, mainly Labour, Scottish MPs.

"We have a fairly collegiate approach; all Scottish MPs are interested in pursuing the best interests of Scotland. So we all get on pretty well behind the veneer. I don't feel - goodness, I'm sitting over here in the corner on my own."

Yet he defends the Conservative corner rather well albeit in an unconventional way. "The practical situation is 420,000 people voted for me to be elected; our Lib Dem friends got 460,000 and they got 11 MPs."

That sounds like an endorsement for proportional representation, the Tories' electoral friend in Scotland.

"I'm not advocating proportional representation," insists Mr Mundell, who more than doubled his majority four years ago to 4194.

"But one of the things we don't push enough back on is that I don't represent one 59th of the Scottish population in terms of the vote, I represent one in six voters.

"Yes, we have been in a difficult and challenging position and the Nats try to say that we don't have Tories in Scotland; well, yes we do. One in six people in the UK General Election voted Tory with just slightly less in the Scottish parliamentary election when I got 15 colleagues. Conservatives are involved in running a number of councils and are the biggest party in two or three councils. So there is not not a Tory Party in Scotland."

Yet why is so much of Scotland anti-Conservative? "I don't believe it is a Scottish thing as sometimes portrayed. A lot of the dynamics are at play in northern cities and some areas in London. There are specifics in Scotland and there were periods where we did not manage the brand very well. We upset lots of people who should be natural supporters."

So if there is a secret band of Conservative groupies in Scotland, why don't they vote Conservative?

Mr Mundell refers to polling by Tory benefactor Michael Ashcroft which, he says, shows it has nothing to do with the late Margaret Thatcher or the legacies of the past but that potential Conservative supporters simply see a Tory vote as a wasted vote.

"That has become a self-fulfilling prophecy. They think if you vote Conservative, you won't get a Conservative elected and so a significant number of people have made the choice of 'least worst'. So there is a group of people who vote SNP because they don't like Labour, people who vote Labour because they don't like the SNP and there are some people who have voted Liberal Democrat."

So how do the Tories get over this fundamental hurdle? "What we have to deliver is the message that we are relevant, that we are talking about the things that are of relevance to people in Scotland. That we aren't some fringe party. If you vote Conservative, you can elect Conservatives."

Indeed, the father-of-three believes the referendum campaign is helping revive the Conservative cause in Scotland and could help swell the ranks of Tory MPs north of the border come the next General Election.

"We are very clearly identified as making a passionate case for the continuance of the UK and in particular what we hope is that people who are currently voting SNP but who are Conservative in their outlook will return to the fold because they will understand that I'm not just voting at this moment to increase the number of police on the beat or to freeze my council tax, if I vote SNP, I'm going to constantly be in a constitutional confrontation."

Mr Mundell admits the Tories made some bad mistakes in the past but he, David Cameron and Ruth Davidson are keen to avoid repeating them.

"The key mistake was not to oppose devolution; people did that on a principled basis. The mistake, which has cost us support, is that even when it was clear that was the majority view of the people of Scotland, we blocked it."

The Scotland Office Minister makes it plain that even if, after a No vote, proposals on further devolution do not meet with the Tory approach, his party will not seek to block them. Indeed, Mr Mundell appears clear that the next time the devolution train sets off for a new destination the Scottish Conservatives will be on board.

He speaks of the "possibilities for devolution" and declares: "Clearly, there's scope in relation to income tax." He insists his party does "not want to rule out anything in relation to non-financial powers" and as a party committed to localism "we should be in the vanguard of proposing as much devolution and fiscal authority to local government as we can".

But one vanguard the Scottish Conservatives will not be in is devolving corporation tax. Referring to the US states, the frontbencher says the American experience shows having competing corporation tax rates simply leads to "a race to the bottom".

Lord Strathclyde, the Tory grandee, is drawing up the party's next move on whither devolution and will produce his recommendations in late May. Mr Mundell is adamant that this time round the Tories will not be standing in a corner on their own.

"The Scottish Conservatives will not be bystanders. We will be there and have an offering on the table, around which to formulate discussions. What we are not saying is that we are going to persevere with our offering if that is not acceptable to others within Scotland. We are not going to enforce our settlement; we want to work with others to get an agreed settlement."

But while the Tories might have ruled out a pro-Union alliance ahead of September 18, it seems clear that on the back of a No vote they intend to play a full role in discussing what extra powers can be devolved to Holyrood.

"After September, you would be looking at how the parties would be ensuring they were able to set out a process by which the different approaches are taken forward to achieve a consensus."

This, he admits, could be in the form of a UK-wide constitutional convention or, in purely Scottish terms, a Calman Mark II process.

"It's inevitable post the referendum and post the General Election, clearly there is going to be a process by which the different proposals are evaluated and a common agreement is reached."

But there is the little matter of the referendum to be got through and Mr Mundell believes Alex Salmond is making some fundamental errors.

On what many see as a hostage to fortune - the First Minister's declaration of March 24 2016 as Independence Day in the event of a Yes vote - the Scotland Office Minister appears clear that the timescale is wholly unrealistic, has "no legal status" and is just "an aspiration".

"It's only achievable if he was willing to make huge concessions on what his position is. Either he is immediately going to throw the towel in on a whole range of issues or it's simply not achievable."

He goes on: "It's going to be more than 18 months if there is going to be meaningful negotiation on significant issues. I don't suggest it's a direct comparison but many people who have been through a divorce know that 18 months can be quite an optimistic timescale to get through that; that's just two individuals trying to disentangle their lives. It can only be achieved from very significant concessions."

On EU membership, Mr Mundell believes the SNP Government would win favour with voters if it accepted there would be challenges ahead. "But it's always: 'This is going to be automatic. President Barroso is wrong. We have found an assistant professor at the University of Riga who has a different view and that view will prevail over the President of the European Commission.'"

On Nato membership, he dismisses the Nationalist position as "absolutely incoherent and incapable of being sustained". He explains: "Nato is a nuclear alliance. You cannot join Nato if you are not willing to sign up to a nuclear umbrella. Either Scotland will sign up to Nato within this 18 months on the basis it's a nuclear umbrella or it won't be in Nato. That's the reality."

And on the currency union: "There is not going to be one; that's absolutely clear. It's a fact. It's part of the facts on which people will have to form their judgement. People will find it very difficult to vote Yes if they are not clear what currency Scotland will have after the referendum. That's a huge leap of faith."

While the referendum dominates Mr Mundell's life, admitting he is eating, sleeping and breathing it, there are times when the reality of the lives of others not in the political bubble brings him up short.

"I knocked on the door of someone on Saturday, who did not know there was a referendum. That even took me by surprise."