The definitive answer is in LASORS, which you can download for free on the CAA website,
www.srg.caa.co.uk
Basically you have to fly 12 hours in a two year period but 12 hours have to be completed in the final 12 months of the 24 month period, (don't ask!) During the second year you have to complete a dedicated check with a flying instructor of at least 1 hour. The re-validation has to be signed off before the expiry date. Also to carry passengers you must have been the sole manipulator for 3 take-offs and landings in the preceding 90 days. Many flying clubs require you to fly their aircraft at least every 28 days anyway to remain current.
Like I say above, check out LASORS for the precise answer!