As a holder of the old CAA PPL rather than the newer JAR-PPL, if you pass the medical there is a possibility that your PPL was originally valid for the holders lifetime. Meaning that even though you have not flown for many years you remain the holder of a UK CAA PPL but with no valid class or type rating. Check in Lasors but if this is the case you will probably just have to renew your single engine piston rating (assuming this is what you wish to fly) and there will be no specified minimum training required - just what your instructor deems is sufficient to achieve competency and pass the SEP flight test.