vfrrider481, sorry to hear about your medical situation.
Your first port of call should be your GP. Only a GP may countersign a Medical Declaration which is necessary for NPPL-level flying.
Assuming he/she is happy to countersign your Medical Declaration, the next thing to consider is the expiry dates of:
1. Your SEP Class Rating.
2. Your CPL.
If both are currently valid (apart from the JAA Medical Certificate), you can continue to fly SEP aircraft restricted to SSEA privileges under the conditions of ORS4 No.816 - and you don't even need to pay anything (except perhaps your GP) nor contact the CAA!
However, when your SEP Class Rating meets its validity expiry date, it will need to be replaced with an SSEA Class Rating for which you must apply to the CAA - and which will attract a fee. You can then continue flying using your CPL and Medical Declaration until your CPL meets its 5-year point. At that point, you will need to apply to NPLG Ltd for a NPPL.
The whole idea of ORS4 No.816 is to help people, in situations such as yours, to continue to fly without additional requirements whilst their medical situation is reviewed. With luck you may get a JAA Medical Certificate back again and continue - if not, then there's the CPL/SSEA then NPPL(SSEA) option.
Good luck!