You have two options:
a. you must either have 12 hours flying in the12 months preceeding the valid until date (expiry) to include:
6 hours PIC
1 hour Dual Instruction signed by a JAA instructor
12 take offs and landings
or
b. pass a licencing proficiency check with an examiner in the 3 months preceeding expiry.
Both a) and b) must be signed up in your licence by a JAA Examiner who completes LST/LPC form SRG1119
http://www.caa.co.uk/docs/33/FORSRG1119Iss6.pdf and sends a copy to the CAA. In theory it could be signed by any JAA examiner however, they will not necessarily have the correct form or know where to send it.
You can revalidate by experience at any time in the 12 months preceeding expiry and get another 24 months starting from the expiry date.
If you go one day over, the rating expires and you have to take a Skill Test to get it back. The content of the Skill Test is exactly the same as the Proficiency Check at b.
Details in LASORS section F1:
http://www.caa.co.uk/docs/175/Lasors_Section_F.pdf