As far as present value is concerned your best bet is to check recent U.K.auction results for a similar gun[ie;past 2 years max]. Holts auctions would be a good starting point. Better still send Holts details of your gun. I am sure they will provide a pre-sale estimate.
I am aware that a similar 28G gun by Watson, recently changed hands in S.W. Ontario, reputedly in the $5-6000 range.This is only the second 28 G that I have seen in 40 years attendance at Ontario gun shows.

Roy Hebbes