Schools generally hold certificates for 5 years - have you tried the school's exams officer? They will also be able to tell you what happens if they have been 'disposed of' (they may be returned to the exam boards).
I had a friend who contacted his old school after 30 plus years and YES they still had them. Though he wished he'd never bothered as they were all Ds and Es and unclassified. He used to tell me he was quite clever at school as well hee! hee!