Depends on condition and which model it is. Could be £30, could be £110. Most go for £40-£70.
A Mk I, which has a metal end cap, grooves in the front part of the stock and four scope grooves might be worth the most.
They are efficient little rifles and have nice barrels. The trigger is unsophisticated, but they are generally well regarded as plinkers and ratting guns.
Have you a picture?