Yesterday I sent off my UK JAA PPL (A) for renewal as NPPL,the forms are on the NPPL website, and John Pett was most helpful.
I'm not sure if that helps you,I believe you can't fly a light aircraft on a commercial licence,or maybe you need a check out with a CFI.
I know one of my airline pals had to go through some checks to get back to GA.
Just noticed you have SEP,as long as you can get a medical declaration you should be OK
Good luck
Lister