Hi Mad-Air
I think you already have the answer to your question. You need to pass a proficiency check with a flight examiner to renew your sepl rating. Prior to the check flight do as many hour's as you and your instructor think necessary. The check flight will be similar to a GFT so practice stall's, steep turn's, PFL's and circuit's.
If you convert your license to JAA you can still attach your IMC rating to it after renewal but you can only use the IMC in the UK. The IMC renewal is another flight test. Your night rating, I think, comes with the sepl renewal but you need to do 1 take off and and landing at night (every 90 day's) in order to carry passengers.
One last thing. Be careful when you log p1/s. As far as I know you can only do this for a successful flight test. Without a current license you need to log p/ut.
Hope this help's.