I'm assuming that you have a UK issued JAA Licence.
You cannot 'revalidate' a rating once it has expired. You have to 'renew' it.
To do so, you should visit your local ATO/RF (Flying School) and have the HOT/CFI asses how much, if any, training you will need to pass the Flight Test (PC)
Once you have completed the required training, (if any) and been issued with a Course Completion Certificate(CCC), you then take your CCC to any EASA PPL Flight Examiner(FE), and complete the PC.
If the FE is not authorised by the UK, he/she will have to complete the UK National Briefing from the CAA.
You are then good to go until your JAA Licence expires on 26-8-16, by which time you will have to have your licence re-issued as an EASA one.
MJ
Ps. For '8th April 2015', now read: '8th April 2018'.