Here's where my lack of knowledge comes shining through...
I spoke to the bloke selling it today and had a chat with him. It turns out it isn't an R65 after all. It's an R45. Here's what it looks like:
He bought it as a project, intending to convert it into a GS, but hasn't got round to it. Looking at some of the amazing Guzzis in his garage, I can see why his attention has been drawn elsewhere. It isn't MOT'd, but starts and runs well. It seems in pretty good nick, some rust on the cycle parts, but nothing outrageous.
I'm seriously tempted. If I can knock him down by £100 or so, maybe I'll even be able to get it without the need for a divorce. I'll go and see him tomorrow and look through the paperwork - he says there is loads - and talk about a price.