What Factor30 means is that you'll need to know which body set the questions, marked the answers and awarded the grades. If it CSE, rather than O-level, tracking down the body which has succeeded the relevant CSE examination board could prove to be rather tricky.
As an example, the school I taught in (in Sheffield) used TWYLREB (The West Yorkshire and Lindsey Regional Examinations Board). Their records are now held by AQA (the Assessment and Qualifications Alliance) and that's who past pupils from my school would need to contact.
Other CSE boards, and their successor bodies, are listed in this AQA document:
http://store.aqa.org....y/AQA-EXAMRECORDS.PDF
Chris