Revalidation (of your Class Rating) has to be signed off by an Examiner, however the flight (and the signature in your log book) can be done by an FI or a CRI (Class Rating Instructor). So it's two signatures, one in your log book and one in your licence, and they must BOTH be done BEFORE expiry. It's no use turning up one day late to the examiner with your log book and saying "I did the flight two weeks ago , here's the signature, can you sign my license please?"
No cheque to the CAA (for a change)
Look at
LASORS 2006 for the CAA interpretation of the JAR's or at the
Regulations themselves for the real McCoy (CAA somehow seem to find it very difficult not to embellish the requirements)
Mike