Rob - British mandoline banjos have never really been in demand, so it's not going to be worth more than a couple of hundred, and probably a lot less. If the head's just sagging a bit, tighten it up - go round it with a spanner, just giving a quarter of a turn to each nut, and check it each time you get right round. It's abouit right when it looks tight but you can still push it in a little with your fingers. If it needs a new head, you can get a vellum from Sully at Halshaw Music, if you're in the UK. He sent me one by return of post. You need one about 4 inches wider than the rim of the banjo.
Cheers
Ray