Given the low mileage, one owner from new, top spec, good service history, plus the all important belt / tensioner change, this car has a lot going for it.
Despite all this, I'd be surprised to see this S2 Turbo getting much more than £1200 - £1500.
The S3 Turbo is the more desirable version and a similar S3 would be pushing closer to £2000. The engine of the S2 Turbo is a bit peaky and needs to be kept high in the revs to keep it in the fat part of the power band. Both the S1 Turbo and the S3 Turbo make for more "puntable" B-road cars.
As a bench mark, 5 yrs ago I sold a very clean rust-free S1 Turbo with black 8.32 leather interior, FSH and only 2 owners from new for £1200. And I didn't have a queue waiting at my door.