You probably didn’t read or at any rate give much credence to the Sunday Times or Guardian pieces, or the even less nuanced Twitter summaries, that suggested that Liz Truss, the foreign secretary, “insisted on an obscenely expensive lunch (£3000!) … [for which] You picked up the bill”, as Larry the Cat (@Number10cat) put it.

But if you had, in passing, noticed something along those lines, you could easily have thought, as any sensible person might, that it was an outrage along the lines of refurbishing your duck house with Colefax and Fowler wallpaper and marble tiles, and been the more appalled to think that it was ultimately paid for by Gary, who stacks the shelves at Asda on a minimum wage zero hours contract.

Naturally, that’s exactly what dunces who populate social media or, in mainstream outlets, fill up the comments without bothering to read the piece or think for two seconds, did. For good measure, and nakedly political point-scoring, so did Labour frontbenchers such as Angela Rayner and Emily Thornberry.

Quite a few people seemed to have concluded that this was a meal for two, or made the stellar point that you can get a fairly substantial meal for six for £20 at McDonald’s. All twaddle, of course.

For a start, the total bill was not £3,000 but £1,400, with service and VAT accounting for the usual chunks of that. For 10 people. In Mayfair, probably in the top five of the world’s most expensive bits of real estate. With the US trade representative and her team – a reasonably interesting group, you may think, for the Foreign Secretary to be buttering up in a post-Brexit climate.

The suggested alternative (presented by civil servants before the lunch) was the Soho restaurant Quo Vadis, which quoted £1,000 up front – actually £250 more, as it turns out, than the food component of the bill at Ms Truss’s venue, which was a private members’ club, and thus avoided all sorts of security costs and privacy considerations.

As it happens, despite being a journalist, I’ve always been fairly sceptical about the value of expenses-driven hospitality, and hardly ever put in for them myself. (This was frankly idiotic, because for years it meant I was effectively subsidising Lords Black and Rothermere, and various other folk who earned more a day, and probably an hour, than I did in a year.) But if you despise the whole notion of the business lunch on the grounds of socialist purity or moral rectitude, you may as well ask about 65 per cent of the hospitality sector to pack up and go into some other line of work.

During that period, however, I also shared a flat with people who worked in the lower reaches of the restaurant trade, and so I know a little about the economics of that. It strikes me that the cost of Ms Truss’s lunch for 10, while it could certainly pay the supermarket bill for that many families of four for a fortnight, or buy an awful lot of cheeseburgers, was not especially expensive for a meal in central London.

How much does it cost to make, rather than buy, a meal in London W1? Well, small to mid-sized restaurant premises there seem at the moment to rent for about £10-20,000 per calendar month, and I assume that’s cheaper than usual, given that no one has been able to make any money out of hospitality for two years, and no one in his right senses would be going into it now.

That’s always the case, actually, though it doesn’t stop restaurants and bars being a very popular choice of start-up business; few going into it seem to have absorbed the well-known fact that almost 70 per cent of such new enterprises go bust within a year.

Anyway, let’s say you can come up with about £200,000 for the rent. In somewhere like Mayfair, that’s maybe 15-20 per cent of your operating costs. All you have to worry about now is business rates, insurance, liability, set-up costs (£300,000 is regarded as low to average for a mid-sized kitchen), other construction (£300-£500 per square foot), licensing, staffing costs (on which you’ll spend at least 35 per cent of your gross sales, in an upmarket place), pensions, marketing, website, furnishings, cutlery, crockery and linen.

Then you’ll need to buy some food and drink to cook, serve and sell. Call getting that in about 40 per cent of what you can get out of your customers. It would probably be half that in a cheapo pizza joint somewhere outside zones one and two in London, but the US trade delegation tends not to go to those places all that much. Somehow you’ve got well above 90 per cent of what might eventually come in (if you’re in the 30 per cent who don’t more or less immediately go to the wall) going out before you’ve even decided what to call the place.

I’m not suggesting that £75 a head, and the same again for drinks, service and all the rest of it, is the kind of money most of us can often lay out for lunch, or would want to. But it’s hardly out of line for normal expenses for a business lunch in the middle of one of the world’s most expensive cities – the civil service’s suggested alternatives would all probably have been more. More to the point, it seems equally astonishing that anyone can afford to run a business providing a fairly swanky meal for that sort of money.

I don’t have that many recent points of comparison since, like most people, I’ve not been going out. But I’ve had two meals in London in the past six months. One was at a take-away in a dingy bit of SE5, where two burgers and fries and two cans of ginger cost £32. The other was the Prime Minister’s celebrated dinner at the Garrick club, for former Daily Telegraph leader writers, which also caused a lot of confected outrage.

I paid my own way to that, too, and – although club food tends to be half the price of restaurant equivalents in the same part of town – it was £80 a head. There may be a case for saying ministers and MPs shouldn’t have business lunches at all. But you can hardly complain about Ms Truss’s one being notably extravagant.

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