Hi Aerofoil,
My understanding from Lasors is that the rating is valid for 12 months and you can revalidate by doing a proficiency check within the three months preceding the date of expiry. However you also require 10 sectors flying experience and if you dont have this then you have to do one sector with a CAA examiner. This can be part of the profeciency test.
I am posting the link to Lasors and the section you require is F4.
Regards
Willby
http://www.caa.co.uk/docs/33/LASORS.PDF