CAP 747, Appendices 1-3, GR 24 - my documents are dated 2005 so things may have changed but I doubt it. You can go to 12 years+20%. Doesn't matter however if you're not going to hire it out. You can just keep going beyond that.
Gipsy Major engines not applicable (in fact it may be all Gipsies) as the manufacturer didn't publish a service life! So if you're looking at a Chipmunk you don't need to bother (yet - I'm sure it's only going to be a matter of time
)
I'm sure proper engineers will be along to correct me if I'm wrong - I've just seen it from an owner's perspective so far.