Is there anyone in London with a 5x4 enlarger who can help me make some 10x8 film positives from some 5x4 negatives? The only commercial outfit I've been able to find who will do this uses a digital intermediate step which I'd like to avoid (and they charge a small fortune too).

Just a suggestion but have you thought about asking your nearest art college - they may have a 5 x 4 enlarger and one of the students might be willing to do it for you.

http://www.thecameraclub.co.uk could be a possibility.

whereabouts in London are you? I have a bench 5x4 enlarger you can borrow.