From Lasors 2008 (New version due out soon)
See page 259 - Section F (I happened to be looking at this myself the other day)
Revalidation by flying experience
A SEP (Land) and/or TMG class rating can be revalidated by flying experience by producing logbook evidence to an appropriately authorised JAR-FCL Examiner, before the rating expiry date has passed, of the following flying experience completed within the 12 months preceding the rating expiry date.
12 hours of flight time in SEP or TMG aircraft as appropriate to include;
i. 6 hours as pilot-in-command;
ii. 12 take-offs and landings;
iii. a training flight of at least 1 hour’s duration with a FI(A) or CRI(A)* who must countersign the appropriate logbook entry (see full details below).
*The instructor must be authorised in accordance with JAR-FCL to instruct for the JAR-FCL TMG or SEP(Land) rating as appropriate. UK QSP only may undertake the instructional flight with a UK Military instructor. This training flight may be replaced by any other aeroplane proficiency check or skill test for an instrument, class or type rating (as defined by JAR-FCL) with a JAA qualified Examiner, or by a flight test for the issue/revalidation or renewal of a UK IMC rating.
• If revalidating by flying experience, and providing the examiner signs the Certificate of Revalidation page within the 3 months prior to the rating expiry, the validity of the revalidated rating will be calculated from the date of expiry of the preceding rating.
Important Note: If intending to revalidate by flying experience, the Certificate of Revalidation must be signed before the expiry date of the previous rating has passed, otherwise the SEP/TMG rating renewal requirements of F1.5 will apply.
So, you will probably find that you still need a training flight as this has to be with 3 months of the revalidation. Good luck, hope this helps. (You can download the full Lasors as a PDF for free from the CAA site)