USP It is Britain's only natural thermal spa, making bathing and water treatments a unique experience. The stellar attraction is the rooftop pool, where you can wallow in the water on "bubbling air seats" as the turrets and towers of the historic city jet up on all sides around you. These are best enjoyed at night, when the skyline is floodlit, the temperature drops, and the thermal water begins to steam.

QUALITY OF EXPERIENCE I tried the Thermae's signature treatment, Watsu, which is a rather astonishing combination of massage, t’ai chi and regression therapy. The therapist fastens luminous pink floats around my ankles and invites me to close my eyes and drape myself backwards into her arms. Soon she is swooshing me gently through the water, my head cradled on her shoulder, my limbs gone to liquid, my ears submerged, so that I can hear both of our heartbeats. She says. "It’s like being back in the womb." She's right. The intimacy of drifting about in a stranger’s arms is liberating. A full hour of this and I’d be dribbling and calling her "mama".

AMBIENCE History dictates it will be a "people’s spa", tidy and simple. What you won’t get at Thermae, for all its £35m spending splurge, are five-star trimmings. So no deep-pile carpets, no gold taps and no post-massage fruit kebabs. Architecturally, Nicholas Grimshaw’s New Royal Bath building is a hotchpotch. Think Doctor Who visits Jane Austen. Outside you'll see a glass cube rising above the facades of the city’s surviving Georgian bath houses like a giant Glacier Mint plonked into a toffee assortment. Inside the ground floor is dominated by the Minerva Bath, and it is joltingly modern — a curvilinear inkblot of aquamarine water supported by vast white pillars shaped like upturned ear-trumpets. Yet visit the original Georgian whirlpool and suddenly, you’re in ye olde Bath again: the golden 18th-century archways remain intact.

IN CROWD Modern day Romans from the historic city; mums and daughters, couples and, most likely, some aggrieved Somerset council-tax payers who have had to pay for construction cock-ups and endure repeated delays for the opening, looking for pay-back.

FOOD An onsite cafe/restaurant featuring the cuisine of Michelin-starred chef Martin Blunos has a choice of a couple of light healthy main meals, soup, smoothies and even red wine. Too bad lunch finishes at 2.30pm.

WALLET WATCH A two-hour session costs £19; four hours £29; full day £45. Treatments are extra, and start from £36 for a 30-minute thermal hydro-massage to £55 for Watsu. Various day packages are available. Local residents enjoy a 15 per cent discount. If you’re coming only to bathe, don’t forget to bring a towel, robe and slippers (or else you’ll have to hire them at £7.50 a pop).

NEED TO KNOW Thermae Bath Spa. The Hetling Pump Room, Hot Bath Street, Bath BA1 1SJ (01225 33 1234; www.thermaebathspa.com)

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