Thanks all for you input. Having knocked back a few whiskies I plucked up the courage to ring this man up. I turned him down, politely. We had a chat for about fifteen minutes and then he rang off. He was really nice, funny and sort of unassuming - just a normal bloke. He seemed totally legit and all the facts he gave me stacked up. I still don't know a great deal about him - like what he looks like! - but definitely not an axe-wielding monster. He spoke to me respectfully, and we had a proper two-way conversation, and he was interested in what I had to say. I have not had a conversation with my current partner like this for ten years or so! I actually felt that my opnion was worth something, and for the first time in ages felt alive and vital - just like the woman I used to be, in fact, and not just the bloody au-pair or something!
On the plus side, at least I know that there ARE nice blokes out there, and should the opportunity arise again, I will think VERY seriously about how I respond to a similar invitation.