Hi grimreaper and welcome to the BBS. The Diana 25 was a junior rifle produced by the Diana, Original Diana (name speaks for itself) and the Milbro Diana company who acquired use of the name and produced rifles in Scotland. It has been best calculated that around ten million of these Diana 25s were made between them so they are not a rare item, although they are very fondly thought of. I have copied some links for you below about these rifles to help. So to value. If very good condition expect to get around £50 (tops), good condition between £30 and £40, fair condition £20 to £30 and in poor condition? Well these get broken up for spares or given away to project builders (see first link below). Atb & HTH: Gareth W-B.