Thanks for the advice guys.
Lowtimer - Yes, Bourn's quite close to where I am, although I'm planning to complete my PPL at Gransden for the time being. They've got very good rates for their Cherokees and C150. Once I've re-qualified I'll probably stay with them as I flew there when I qualified the first time.
Back on the subject of Chipmunks, I must confess the one I really have got my eye on is WB569, currently sitting in hangar 2 north at Duxford - had my first ever flight in that one as a 13 year old cadet, and so I was overjoyed to see that she's still flying today. Now just got to pick the right six numbers tomorrow night and make her owner an offer he can't refuse...