12 months is ok. Extract from email from CAA SRG....
"You are also correct in identifying the fact that, provided the examiner signs up the Certificate of Revalidation page within the 12 months prior to the expiry of the current Certificate, the revalidated rating will run from the date the existing rating expires, even if the current rating has more than 3 months to run. (this does not apply if revalidation was by proficiency check)."
Cheers
EA
