As you know the date and place of your exam, and it was after 1993, you will be able to get confirmation of your marks for free, and a replacement certificate for a cost. If you look
here you will see under regulation 16j the information that they need in order to find you a new certificate [I have linked to the regulations for outside of the UK, as that is where you took your exam].
You can contact your AB local rep via
this page (again, I've linked to non-UK reps) who may be able to provide more advice such as how much it will cost, or alternatively go straight to AB London via
this form