Need a picture or a more detailed description, could be between £100 and £400 depending on what you consider to be 'good condition' and which particular variant it is. There's three on guntrader at the moment, between £225 and £285. Maybe compare yours to those? Bear in mind GunTrader prices are inflated about 30% compared to the enthusiasts on this bbs...

https://www.guntrader.uk/guns/air-ri...01019205922027

https://www.guntrader.uk/guns/air-ri...01019203629025

https://www.guntrader.uk/guns/air-ri...01019203357024