I think you're right Pete but for a diffrerent reason. For me the sales are less important that the fact that demos were usually stored away after a short run of plays on radio stations or even no plays at all in record store. Issues however were bought, granted sometimes in low numbers, to be played and were often not cared for, stuck on those 60's auto changer spindles with 9 other records, played with by kids all the time the demos were sitting neatly filed in a archive somewhere. No wonder there are more demos in playable condition than issues. IMHO
I'd give £200 for a demo and nearer £400 for an issue but I'm not really a British collector they are a funny breed.