Hi Exmax,
I had to do this aswell. It is quite easy. You need to call the school or college that you took the GCSE's at and they will give you a telephone number of a company that holds all their certificates. You have to have the school/college's registration number (or something like that!).
When you ring the other company they will ask you to fill out a form telling them all about yourself and when you took the exams. It cost me £18.00 and only took 1 week to receive them back but they did say it can take upto 31 days!!
Good luck.
L.xxxx