Ta-da!
http://www.lewisham.gov.uk/myservices/tr...ssons.aspx
Yep, lessons from scratch, or for those who just want to get braver at being on the road, I think. And on the left there's a link whereby you can borrow a bike for a month for a tenner...
Now, I must say I got through to them through another link and they've not got back to me yet, so I don't have any actual experience of how well this whole thing works, but... gotta be worth a try!
Edited to add: so I just followed the link, and you give them loads of info about what you can/can't do on a bike, so they can judge your level of experience. You can also borrow a bike for the lesson. If this comes off I will be WELL IMPRESSED.