At the risk of being corrected....
Your JAR SEP (single-engine piston) rating is valid for two years. Ignoring passengers, and assuming that you don't want to revalidate via a Proficieny Check with an Examiner, you need 12 hours logged in the final 12 months. There are some requirements as to what these hours must include - rather than try and remember them, I'll point you to the
CAA website or
Irv Lee's FAQ 
If you would like to carry passengers, then there's also the "90 day" rule - no passengers unless you have made 3 take offs and 3 landings in the last 90 days in the same class/type. If you're doing it at night (i.e. you have a night rating) then one of those three must have been at night.
Finally, your club may have additional requirements - e.g. a "60 day" currency rule.
Hope I haven't left anything out