I agree.
If it has all the bits, then £1500 is not out of this world, even for such an oddball calibre, although, being a muzzleloader, is not that important.
In these cases, condition is everything.
Is it in proof? If not, factor that into the deal - figure on £100 or thereabouts. Is the vendor selling it a a shooter or a wall-hanger? If as a shooter, then HE is responsible for the proof, not you. It is an offence in UK, and any of the other CIP signaturee nations, to sell a shooting gun that is not in proof.
tac