Your CAA PPL(A) is still valid - it remains valid for life. What has expired is your Single-Engine Piston (SEP) Class Rating; to renew this, you will need to attend an Approved Training Organisation (ATO) and complete sufficient refresher training to pass a proficiency check (content of the prof check is at CAP 804, Section 4, Part L, Page 51). The amount of the refresher training is at the discretion of the ATO (or RTF until April 2015). Before the CAA will renew the rating, they will require that you also hold at least a Class 2 medical (no such thing as a Class 3 these days).
Once the class rating has been renewed, you're good to go with full PPL privilege in any SEP aeroplane until 8 April 2014. Thereafter, you can fly EASA aircraft (i.e. those with an EASA C of A) only with the privileges of a LAPL(A) (aircraft below 2000kg with no more than 3 passengers) but you retain full PPL privileges on 'Annex II' aircraft (mainly historic aircraft, homebuilt, ex-military, microlights, etc.).
To retain ful PPL privileges on EASA aircraft after 8 April this year, you will need an EASA licence (see CAP 804, Section 4, Part P). This does not mean that you have to give up your CAA licence as you can hold both and it's probably a good idea to do so.