Treat-ment: The Dorchester Spa's Panthermal Bath

Time taken: 1 hour

Cost: pounds 42.

Why have it: You're jet-lagged, puffed up, overworked and run-down.

When to have it: Any time you want to feel wide awake.

The Dorchester Spa is seriously smart. From the hotel's reception, stairs and corridors of deep-pile carpet lead to what looks like a scene from Poirot (if ITV had a pounds 3 million budget, that is).

Glossy women recline on butter-coloured sofas, art deco sculptures fill spotlit recesses, mini-fountains spurt from ebony water-lilies. A series of rosewood doors leads to a labyrinth - even the therapists get lost - of chic little treatment rooms. Highlights include an Ionized Relaxation Room (like a '70s sex den - all water-beds and violet fluorescent light), a hydrotherapy room with a bath the size of a small speedboat, and the swishest changing rooms you have ever seen. Everyone can use these; even if you've rushed in from the street and booked a half-hour massage, you'll get your own key to a wooden locker filled with fluffy white towels, slippers and a man-sized bathrobe.

The Panthermal Bath - basically, sitting starkers in something that resembles an iron lung - is exclusive to the Dorchester and is one of their most popular treatments. It begins normally enough with the therapist rubbing exfoliating handfuls of sea-salt on to your body as you lie on a thermally heated treatment bed. Things only get weird once you're in the Panthermal Bath. For a start, it's not a bath at all, more a giant egg. You sit inside on a sort of slatted wood deckchair with your head sticking out the one end. As you try to relax, the wooden slats digging painfully into your bottom and you are hit by waves of ionized steam. You begin to sweat a great deal. The therapist explains that she will be staying with you for the next 30 minutes because first-timers are often disturbed by the bizarre noises the Panthermal makes. Right on cue, there's a rude, sucking noise followed by a high-pitched whine as the bath fills with pure oxygen ('to feed your tissues and get everything going) . After the oxygen, comes a skin-softening mist of aroma-therapy oil and water (lemongrass flavour 'to uplift).

The penultimate stage of the Panthermal is supposed to be refreshing; but punishing is the word more likely to spring to mind as needle-sharp jets of cold water criss-cross your body. The therapist will make sympathetic noises and reassure you that the water is about to turn warm - it never does. Feel free to yelp; your suspicions that you are being seriously damaged will be confirmed when you step out and see your thighs striped with angry, red welts. The final massage with Thalgo body lotion goes some way to compensate, but what will keep you coming back for Panthermals is the caffeine-like energy surge you'll have for hours after.

Charlotte Fidler

The Dorchester Spa,

The Dorchester Hotel,

Park Lane, W1

(071 495 7335)

(Photograph omitted)