As Thumpango says, it has apparently recently been changed to 18 months from the date of the first pass. I hope this
is the case because it's just over a year since I did my air law and I'm about to take the GFT on the back of this.
Anyway, here's the link to the CAA doc. At the top of page 4 in
this doc (PDF) it 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 the theoretical knowledge examinations.
Sounds pretty conclusive, but could somebody who is absolutley sure of this please confirm? Thanks.