I suggest that you go to the CAA website and search for Standards Document 11. This details the requirements for schools conducting PPL Theoretical Knowledge Examinations.
Para 15 of this document states that in the event of a student passing a PPL exam, the Ground Examiner must complete the relevant part of the PPL Application form SRG1105A/H, including the number of the examination paper. The idea is that a full record of all exams passed will gradually be built up. If your school has not done this, they are clearly in breach of the Standards Document.
Sadly the Standards Documents are for guidance only and hence not legally enforceable. But the CAA will undoubtedly take a very dim view of the situation if you were to inform them that your school has failed to maintain your records properly. So if you point out to your school that they are in breach of Standard document 11, this will at least give them some incentive to put a bit more effort into finding your records.
You have not helped yourself by allowing this to drift for so long.
The above comment is true, but it is not your responsibility to ensure that your school is maintaining their records correctly.