Wokkaboy,
The details are all in LASORS, the 2004 version of which is on the CAA website - best use a broadband connection though!
Your JAR PPL(H) is valid for 5 years - you renew that by giving a great wedge of unnecessary money to the CAA. If you had a pre-JAR licence it is valid for life at no charge!
However, in practice and in the mean time you need to keep a rating current, in order to be able to fly. A rating is valid for 12 months. In order to keep it valid you need to:
1) have flown 2 hours on the type in the previous 12 months, prior to a licence proficiency check (the 2 hrs includes the LPC flight); AND
2) pass a LPC test on the type (yu can do this up to 3 months before the validity period of the rating and it will count as if you did it at the end).
If you have ratings on more than one type of basic piston helicopter (other than the R22 or R44) you only need to do one LPC, which covers all the types, so long as you have done the min 2 hrs on each type.
That is all the regulatory stuff, but to be safe and not to lose your hard won skills you need to fly more than this, and do challenging flying too that keeps you up to speed. Any outfit that hires you a helicopter should be pleased to help. Almost all such places use a 28 day currency rule, also driven by insurance requirements.