Full answer will be in LASORS which you can find on the CAA web site.
Here's a summary from memory:-
To revalidate you will need to have done 12 hours in the last 12 months of the validity of the rating and 6 of these most have been as P1. You must also have done a certain number of take offs and landings (either 6 or 12, I can't remember which).
You must also have flown an hour instructional flight with an FI. The contents of the flight are often debated on this bulleting board since the CAA offer some suggestion on the contents of the flight, but it isn't mandatory that the contents be anything in particular.
Alternatively, any flight test for the purposes of a grant or renewal of a rating can serve instead of the instructional flight (e.g. IMC or IR renewal).
You can revalidate the SEP class anytime in the last 12 months as soon as you have met the requirements. An examiner autograph is required for the revalidation.
If you go even one day over the validity period of the rating then you'll need to renew it instead of revalidate it. For this you'll need to fly a proficiency test with an examiner. Some people choose to do this for revalidation too since it replaces all of the experience requirements.