As others have said, "training via ATO(RTF) to pass a proficiency check. Then the fun starts! If you have a Part FCL EASA licence, to bring your lapsed SEP rating from the back page requires "licence action" by the CAA.
If you hold a UK or JAR ATPL and the FE can't sign the renewal of the SEP rating, on the ratings page, because the expiry is plus 3 years, you will need to apply for an EASA licence, again because it's "licencing action". (As I understand the latest IN, the EASA 3 year "signing" limit has been extended but only for EASA licences)
Don't forget the April '15 EASA aircraft/licence deadline, unless you only want to fly Annex 2 aircraft. If the CAA are involved in the above remember to submit SRG 1119D or 1107, "course completion certificate" as well as SRG 1119C, SRG1157 and SRG1199. Re the conversion,(SRG1104) if you want to revert to PPL, you will need evidence of your original PPL, if not the CAA should be able to check their archive. Or you can "convert" ATPL to PPL but I can't find the form! Good luck