Alan, give every single detail you have, as Wendy has said, and make sure you put your telephone number down. I always add a paragraph at the end:
'If the details you have in your indeces do not match those given above, then I do not want the certificate as it will not be relevant to my research'.
So far I have over 200 certificates and not a wrong one yet! This paragraph means that if they send you a wrong one you can return it for a refund. I once send for one, to Dorset. They sent me a wrong one, but of the same name, and because of that paragraph, I was able to return it, as it was their fault and not mine. At £9 a time, you just cannot afford to throw it away, unless of course, you are very very rich!!