LASORS Section C (Private Pilot Licence) says...
An applicant shall be deemed to have successfully completed the theoretical examinations for the JAR-FCL PPL(A) when awarded a pass in all of the [above] examinations within a period of 18 months. A pass will be accepted for the grant of a JAR-FCL PPL(A) during the 24 months from the date of successfully completing all of the theoretical knowledge examinations.
Seems to me that everybody is on the 18 month system now. However, if you're in any doubt ask your CFI or e-mail the CAA to get an answer in writing.