Assuming it's a normal SEP class rating revalidation, and not a license renewal, it's free.
Just make sure the examiner signs form SRG\1119 and post it to the CAA. The examiner should also sign your class rating revalidation page that's part of your license.
Most examiners will do this for free or for a very modest fee.
SRG\1119 form here:
http://www.caa.co.uk/docs/33/SRG1119.pdf