A couple of 250s in good condition have been advertised/sold on here for about £150 recently so that could be a guide.

Yours is the later model, available, I think, from the mid-70s right up to the early '80s, with the long front sight rail and stippled pistol grip and forend.

While it has many good points, including the superb quality of manufacture and finish, this rifle can be a pain to service if the hyrdraulic oil-filled anti-recoil damper is leaking, though apparently the damper seals are standard commercial sizes and therefore likely to be available.

Basic service parts such as piston rings, piston bumper and the plastic breech seal may be very difficult to source nowadays, so the precise condition of the internal wearing parts will make a difference to its value. Replacement springs are available from Jgairguns in the US.

Someone might like the look of that butthook, so adding to the value. Also it looks like you have an adjustable aperture/filter attachment of some sort on your rear diopter sight. These can be worth £20 or £30 on their own if sold on the auction site.