When students in the United Kingdom are between 15 and 16 years of age, they take the General Certificate of Secondary Education exam, also known as the GCSE. As a result of the large number of British student who complete this test, each student receives a "candidate number" to identify himself. As you must use this candidate number in any communications with the GCSE council or with the awarding body through which you took the test, it's imperative that you know where to find it if necessary.
Consult any official GCSE materials if you have them. According to AQA, which administers GCSE results, you can find this number along the top of any official GCSE materials under the heading "Candidate Number."
Call your testing centre directly. AQA further states that individual centres, not the GCSE or any awarding body, allocates candidate numbers. When you speak with the representative, make sure and provide her with your full name, date of birth and any other features that may help her look up your candidate number.
Contact any schools to which you have had your results. According to AQA, institutions cannot process GCSE results without the candidate number, so if you've already sent your results off to a school, chances are good that the school has your candidate number and can provide you with it.