If you don't get the 12 hours inc 1 instructor, you can do an abridged skills 'test' with an instructor
Not quite true. As usual, the definitive answer is in LASORS (sections F1.4 and F1.5 to be exact). Here's the summary.
For revalidation (which is what happens before your class rating expires):
a. If you have 12 hours, including one hour instruction, on a SEP(A) in the 12 months preceding the expiry of the class rating, you can go to an examiner, have him check your logbook and sign the paperwork (normally for free), and you're good for another two years. (There are some restrictions towards the 12 hours, but they're usually not relevant.)
b. If you don't have the 12 hours you need to do a License Proficiency Check (LPC) with an authorized *examiner* (not an instructor). This is in effect a mini-skills test.
For renewal (which is what happens if your class rating has expired):
a. If the class rating has expired less than five years, a full skills test is required with an authorized examiner. This will include an oral examination, but the CAA will not charge a fee for the paperwork, since the examiner may sign your license.
b. If the class rating has expired by more than five years, you need to do the same skills test and oral examination, but the CAA will charge money, because in this case the examiner may not sign your license, but new paperwork has to be issued by the CAA.
Obviously you only pass the skills test (or the LPC) if you're able to fly the aircraft sufficiently well. Flying 50 hours or more in a microlight annually will help a lot towards that proficiency.
Note that all this is independent of the renewal of your license itself. You need to renew that license every five years, and you can only renew your license if you have some sort of valid class rating on it. So in your case, you would be renewing your license based on your SEP(M) class rating - assuming that that's the one you're going to keep current.