“Austerity is over” according to Theresa May. I’ve yet to meet anyone who sees eye to eye with her over that. But as we walk into the Register Club, an environment so splendid that it instantly makes us feel like the scruffy poor relations, I realise that for some people, austerity is an utter irrelevance. When one of us asks for Prosecco, the go-to drink of the common people in bubbly mood, our waiter apologises in the most pleasant way he possibly can. “Sorry, we only serve champagne.” That’s Billecart-Salmon Cuvée 2007 at £19 a glass he’s talking about.

Well of course, what else would you serve in such sumptuous premises? Before we entered the bankers-as-enemies-of-the-people era when Fred the Shred hung out here, buildings like these embodied mercantile grandeur, the gilded wealth of capitalism. Now banks feel the heat of customer resentment, they plead poverty, close ordinary branches on the high street and know not to flaunt their wealth. They tout their lavish corporate HQs for sale to corporations, typically chain restaurants; but not in this instance. The 1825 National Bank of Scotland, latterly the Royal Bank of Scotland, has become The Edinburgh Grand, swanky serviced apartments, with a fourth floor bar, the Register Club, that’s open to the public. It is operated by the sole independent, Scottish-owned restaurant group on Edinburgh’s St Andrews Square. Thank heavens it didn’t become a Wetherspoons, the depressing fate that awaits faded buildings of architectural note with major repairs bills looming. So I’d come here for the place alone. It’s like having the most splendid imaginable Doors Open Day.

Take the stairs up if you can, a billionaire’s helter-skelter spiral that’s redolent of Hitchcock’s Vertigo, made all the more striking by a bold, new black and white carpet that’s Greek Key pattern in style. A dazzling geometry that just has to be seen. The Register Club itself is in the former board room, with its stunning American oak panelling carved so lovingly by master cabinetmakers, Whytock & Reid. It’s lit in a kindly way, diffused. You eat in comfy, if ergonomically challenging chairs around low-slung tables; it’s very definitely a grazing place. A restaurant proper is on its way.

Six truffled cheese Gougères are a steal for £6: the choux pastry profoundly cheesy and fired to an immaculate airy dryness; the filling is subtle; truffle pecorino dusted over. A bowl of lobster bisque costs £12 but this brick-red crustacean broth with its taut croutons, its bracelet of rouille, and plump chunks of succulent lobster meat is demonstrably worth it. Personally I prefer it to the New England lobster roll. It’s spilling over with lobster meat, marinated cucumber, and a green mayonnaise that packs a horseradish punch, but I always find those soft submarine rolls too cloying in a savoury context.

Me and the chef don’t see eye to eye on Pissaladière. He’s used a shortcrust-like pastry, piled caramelised onions high on top of it, and used vinegary silver anchovies. In my book it should be more of a bread dough base, there’s a lot less onion, it has to be brown anchovies in oil, and lots more black olives. We’ll have to agree to differ. And its rocket and red onion salad on the side is banal. But I’m totally onside with the crab and avocado atop its homemade, cracker-like crispbread, loving the lightly toasted black sesame seeds strewn over it, the chilli and lime attack that ripples through it.

What might have been a quotidian version of the now ubiquitous Eton mess is rendered celebratory by its champagne jelly. Otherwise we’re talking lightly whipped cream, dry meringue, a few berries. What’s all the fuss about, I wonder? Cheesecake arrives deconstructed. It looks, well, a bit of a mess. But the tastes are good: lemony, white chocolate Crowdie, beaten and airy, gingery chocolate crumble, jewel-like blobs of what might be rhubarb gel.

Like Bolsheviks occupying an opulent Tsarist residence, we lounge about, watch dusk setting over St Andrews Square, hang over the balcony loudly admiring the dizzying staircase. No-one bats an eye. We might as well be millionaires. It’s our place, at least for the span of a meal.

Register Club, Floor 4, 42 St Andrew Square, Edinburgh 0131 230 0571

Lunch/Dinner £25-38

Food: 8 and a half/10

Value for money: 8/10

Atmosphere: 10/10

Service: 9/10

Joanna Blythman is the Guild of Food Writers Food Writer of the Year 2018