1. Obtain a JAA Class 2 medical.
2. Pass a SEP Class Rating Renewal LST with a UK/FE(PPL) Examiner. You will probably need to do some refresher training first.
3. Send to the CAA: Form SRG\1102 (for your licence re-issue), Form SRG\1119 (for your SEP Class Rating renewal) and Form SRG\1187 (for the fees), together with your personal flying log book(s) and old PPL.