The GT Sport is a great version of the QP and can easily command high teens, or maybe even 20k from a specialist dealer with low mileage. But high mileage examples of any QP can struggle. These cars are very mileage sensitive.
A year or so back I sold my standard 2005 QP privately (to Phil) for £15k, but the mileage was a reasonable 50k or so. He sold it on 6 months later for roughly the same price. If anyone thinks they can buy a good (i.e. properly up together with full history, recent tyres, brakes, clutch and service) QP V for around £10k, let alone a rare sport GT variant, they are mistaken. Good early cars, with reasonable mileage, are mid teens. GT Sport versions more.
As far as we can tell, the advertised car is in great nick. But the mileage lowers the value a little. Which brings it back to where the owner has pitched it. I think it's advertised at the right price.
But put it another way. What else can you buy for £15k (approx) that offers the same blend of pace, grace, style and exclusivity? In these terms, it's a bargain.