Thank you for all your advice and suggestions. I had been looking on
ebay too and yesterday managed to find a folding bike quite local. So local, in fact, that it was a first year student at the local university, 5 mins. up the road. His parents had bought him the bike to get around on but he never really liked it and had only used it twice (when I first read that in the description I thought "Yeah right" but it seems to be true as the bike looks brand new).
I collected it last night and hadn't realised that it also had a lock and front and back lights so I have him an extra tenner. He was a really nice young man and I would have felt guilty not giving him a bit more.
I haven't managed to ride it yet - the tyres were flat (I'm guessing from lack of use) - but hopefully it will be okay and for £100 I think I got a good deal: