Hi,
Try looking at the CAA website:
www.caa.co.uk
It may give you info on the PPL. You will need to look at Safety regulation/FCL/Documents. There will be a GID (general Information Document) on the PPL. In here it tells you about the validity of the PPL Exams.
MTPW: I was of the understanding that the PPL exams must all be completed in a period of 12 months from the first pass, and were then valid for 24 months from the date of completion of the examinations.
I would check it, and argue the point. Especially if your FTO are going to try and charge you £10-20 to rsit each exam again.
Let us know how you get on....and good luck!
Barney