They come together at the new Blythswood Square hotel in Glasgow, one of the most impressive British city-hotel launches in recent years (and I speak as someone lucky enough to have stayed at the reopened Savoy).

But back to P. Diddy and that prawn. The rap mogul is just one of several celebrity check-ins since the hotel opened in September last year. I am told, he booked the penthouse, organised a butler and thoroughly enjoyed his stay. I press the staff for more lurid, chandelier swinging details but they are far too discreet.

The Automobile Club connection is that the elegant 1823 hotel building was once its home and memorabilia from its motoring past is proudly on display. The Glasgow to Monte Carlo car rally set off from the doorstep and there are plans to revive the tradition this year.

As for that prawn. OK, it's more like a tiny shrimp but it isn't what I expect to see floating to the surface as I sink into a bath of Hebridean seaweed in the hotel spa. At least it proves the seaweed really is organic.

But it takes more than a shrimp to change my view that the Blythswood spa is quite simply one of the best city-hotel spas in Britain.

The only London one that comes close in terms of space and facilities is at the Mandarin Oriental and that's infinitely more expensive.

The Blythswood spa complex is a "thermal experience" series of rooms and pools built under the ground. As you enter the dimly lit space and hear the water gushing, you immediately start to unwind.

City life is left behind and spa pampering begins. Memoto any other hotelier who offers the odd facial or massage and calls that a "spa" – this is the real deal. If I list the different zones within the "thermal experience", you'll see what I mean: the hydro pool (a giant, walk-in spa), crystal steam room, sauna, vitality pool (complete with hydrotherapy jets to pummel aching joints), laconium (cooler than a sauna or steam room), saunarium (a blend of sauna and steam room) and tepidarium (with heated, curved-stone loungers to fall asleep on – and I did). And I haven't even included the area of treatment rooms.

As you drift from one space to the next, you can feel yourself unwind.

Just the spa alone would have kept me happy for hours but add to that the hypnotic sensation of the crystal steam room(lying on warmed mosaic tiling, gazing at a glowing globe of crystal in the centre of the room), the refreshing effects of the laconium (cool air scented with pomegranate) and the pure joy of letting the vitality pool's powerful jets of water pound my head and shoulders and you can imagine why I'mso impressed.

Many of the treatments at Blythswood champion Scottish ingredients – hence that seaweed bath, for which I sink into warm water tinged brown by the huge bundle of freshly harvested Hebridean seaweed floating in it (along with the odd sea creature).

I'll admit it's not the most pleasant time I've had in a spa–a bit whiffy and slimy – but I'm assured the water contains more than 85 natural trace minerals that could detox and shrink inches, so I persevered. (Not sure about any shrinkage but my skin felt very smooth afterwards.)Other treatments are more enjoyable. A hot-stone massage using milk thistle, cedar wood and juniper oil is wonderfully relaxing, while the warm oatmeal and Scottish herb poultice at the end of my facial has me dozing off.

One final thing to praise before you think I'm on the spa payroll – champagne is served. Watching the groups of mothers and daughters and gangs of friends enjoying a healthy spa lunch in their towelling robes and toasting each other with flutes of champagne, I can't think of a nicer way to spend a day.

Of course, there is more to Blythswood than just its spa. The 100-roomhotel blends the grandness of its 19th-century architectural heritage with some 21st-century design twists – sometimes successfully, sometimes not so. The bones of the building are gorgeous – patterned marble floors worn smooth with years of use, high columns topped with gold, beautiful crystal chandeliers. I love the use of traditional Harris tweed in quirky shades, such as purple and slate grey, on the upholstery, curtains and even lampshades.

But I'mn ot so sure about features such as the dining room's multiple rows of red-fringed lampshades hanging from the ceiling, or the boudoir-like red-velvet booths in the reception area. All a bit fussy.

One very modern touch I've never seen before but which, I think, should be encouraged elsewhere is a tray of Belvedere vodka, a jug of tomato juice and bottle of Worcestershire sauce on the breakfast buffet, for mix-your-own Bloody Marys. Ingenious.

A final slight gripe: I don't love my bedroom. A twin room in the modern part of the hotel (behind the older building that faces Blythswood Square itself ), its ceiling is too low and poky to be truly relaxing.Comfy, yes, but somewhere I want to hang out in, no. Which is a shame, because a tour of other rooms shows they're not all so squeezy. My tip: avoid room205 and treat yourself to room220, the Blythswood Suite, in the old building, with its tweed sofas and black-marble bathroom.

But squeezy rooms, floating prawns and the odd dodgy lampshade are mere blips in the Blythswood experience.

Go for the spa and I guarantee you'll enjoy every moment. Glasgow just got very lucky.

The writer was a guest of Blythswood Square.

Trip notes

Where Blythswood Square, Glasgow, +44 0141 240 1622, [email protected]; townhousecompany.com.

How much Luxury Spa Break from £260 ($406) a room a night, based on two adults sharing a Classic room. Includes full Scottish breakfast, one-hour spa treatment and use of the thermal rooms.

Top marks A world-class spa that makes the hotel a destination in its own right.

Black mark The 19th-century architecture is ruined by slightly fussy modern touches.

Don't miss Give yourself enough time to really enjoy the spa experience. From the walk-in spa to the heated mosaic loungers in the tepidarium, you don't want to feel rushed.