Am I now to understand that if I go up in a club SEP with an instructor and get signed off as a proficient SEP driver, I can then get an SEP signature on my licence every year when I do my combined MEP/IR, even if I don't fly an SEP?
No. Your choices, to get a 2-year SEP sign off, are:
1) Pass a proficiency check with an examiner on an SEP, just as you do for the MEP (but without the IR part)
2) Fly 12 hours in SEPs (with the requisite number of t/os and ldgs, I forget: 6 or 12?) in the year before you want the SEP signature.
Method 2 only works if you keep the SEP rating current. But whereas a bare PPL also has to do an hour with an instructor to satisfy method 2, that's waived for you because you did a skills test for another class. If you keep the SEP rating current, you never have to fly with an examiner/instructor in an SEP.