Picture a six-storey building, translucent and light filled, arrayed with a wonderful range of tiny mosaic tiles in the kinds of shades of lapis lazuli and turquoise that suggest classical Egypt.

On the top storey of the Sheraton One Spa there is an outdoor hydro pool. Gazing languidly across the wide, open spaces around the headquarters of Standard Life and the Edinburgh International Conference Centre, you can contemplate life, the universe and everything. The warm bubbles will embrace you and you cannot but feel that nothing, but nothing, can be this peaceful.

Even in the dark days of winter, with flakes of snow drifting across the horizon, you will feel at one with this world.

Maybe that's why they call it One Spa.

When you manage to tear yourself away from the hydro pool, there is a cornucopia of other delights to explore across the other floors.

First of all is a thermal suite of a calibre no spa afficiando could possibly take issue with. Come into the mosaic enriched serpentine loop of the Rain Forest shower. As your head is delicately spangled with a vaguely botanical-scented drizzle, four other jets warm your body and transport you to some distant Amazonian location.

From there, do the circuit. First is the Laconium where you can build yourself a fantasy involving Roman Emperors while sitting on warm stone seats contemplating the fountain.

The Bio Sauna is a glass-sided room, cooler than a regular sauna, and deliciously scented with a faintly citrus blend which detoxes and relaxes.

The Rock Sauna is for real heat freaks. The thick, dense wood with its roughened surfaces make the walls feel redolent with natural age, and the level of heat is precisely right.

The Hamas Room - we're back with Classical Times - is what a steam room ought to be. Circular, with tiny colour-changing lights embedded in the ceiling, this is perhaps where the colourful mosaic tiles are at their most appealing. In the centre is a quadruple of gorgeous pale green washbasins where you can cool off.

There's also the Aromatherapy room, where a different scent gently steams in each day. You can flake out on one of the heated, contoured relaxation beds after all that hard work, and round off with a peppermint shower.

On your way back towards the changing rooms, you will pass the 19 metre Ozone swimming pool. For someone like me - I react intensely to the chlorine in most pools - this is a joy.

The whole thing is the concept of Susan Harmsworth, herself a glowing example of her own creations. Although she is nearly 60, you would never know it.

She has not only designer and manager of spas all over the world, but she has developed Espa's own range of natural beauty products and she runs a training operation arm too.

The beauty suite at One Spa boasts 11 separate rooms, all of them kitted out to the same high standards as the rest of the centre. Here you can indulge in everything from massage, purifying herbal linen wraps involving hot stones, soothing and nourishing facials and ayurvedic head and scalp massages. Men too, by the way, at least for some of the options.

The whole project represents an investment of some 16 million by the Sheraton Grand Hotel, but Harmsworth's care, quality of design and emphasis upon members and visitors' pleasure is obvious as a result.

Downstairs in the gym, the same light levels fill an area which is spacious, well-equipped, properly staffed with fitness instructors who know their business - including a personal trainer, Ian Bell, available for 40 an hour - and well planned out.

While all of the instructors are helpful and fully qualified, it is probably Ian Bell that makes this gym different. Somehow, he manages to combine motivational skills with a friendly, approachable style that doesn't make you feel as if you are some sort of abject failure. He is straight, honest and understanding and he builds a programme that will test you but not destroy you.

As a full member, you will receive a dinky see-through Swatch Watch and a Tag key. Why? The first is your entrance key. It not only opens all the relevant glass gateways around the Spa, but it also allows you to access a locker in the changing room.

The nifty Tag key has a microchip that carries your entire fitness programme on it. You register the gym, slot your Tag key into the consul, and low and behold up pops your first exercise. As you work your way around the machines, each adjusts to your position and your individual programme specification. You can track your progress (Depressing!) and see how much weight you have lifted (Encouraging! I've hit 5500 kg each session now).

To round it all off, there's an excellent little caf that serves lovely light snack meals and drinks. Down on the ground floor, there is probably Edinburgh's very best Italian restaurant. Santini is the creation of Tony Santini, a Sicilian, and his daughter, and the food that they produce might best be described as wholesome, rustic cooking at its best.

They make their own pasta, serve Parmesan in proper shavings, not little dry grated bits, and their sauces are first class. As far as possible, they source their ingredients from Italy so that they are authentic, and they make their own bread. It's not cheap but it is an unforgettable experience.

It has to be said that One Spa isn't cheap either. My membership costs me 85 a month and there is a one-off joining fee of 250 on top of that. There are cheaper packages available, but they don't include use of all the facilities.

Beauty treatments range from around 25 to 150, and all of the Espa products are available for purchase.

In terms of my sanity as well as my general fitness and well being, it's worth every penny. I'd rather give up buying some new clothes or the odd expensive meal out than give up One Spa.