I've recently been asking a similar queston, as I have an FAA PPL which has expired; my last P1 was in December 94, so take my example as an absolute worst case scenario for yourself.
According to the CAA website....
http://www.srg.caa.co.uk/
... when revalidating an expired CAA PPL, your previous flying training will count in terms of hours flown. As somebody else has said, you'll need to do all your ground exams and the skills test at the very least.
To give you an idea of what I've got to do for a CAA PPL (after having an FAA one expire seven years ago), I'm looking at ground exams, and between 10 and 20 hours (at CFI discretion), covering SSAT / FL, QXC, and the Skills Test.
Hope this helps!