I've been passed a Webley Mark 3 from somebody's loft, and am wondering what it's worth.

It requires some TLC as there is surface rust on all the metal parts and the stock wants rubbing down, but the piston feels strong and smooth (I've no .177 ammo to try it).
There doesn't appear to be a serial number that I can find, though there is a pretty little Webley badge on the stock.

I'm guessing it may not be quite worth trading in against a HW100, but any guide lines would be gratefully received.

Ade.