You could submit a request under the Data Protection Act asking for a copy of all correspondence between you and them in any form, letters, forms, email and transcripts of calls. (Edit – make sure you specify that you want to be informed of emails/call logs etc that they no longer have access to. For example they know you called them @1455 on 1/1/09 but don't have a transcript, sometimes they don't tell you about these.)
You'd then get a copy of the email you need and then you'll have them bang to rights. Refund, interest and a bit of something for the trouble, ta very much. Generally things get sorted very quickly once you start playing this game because it's a massive hassle for the company involved.
It might cost you £10 to cover printing costs etc if they decide to complete the process but tbh they'll cave 99% of the time.
Works everytime for me.
If you don't get the email you want you can complain to the Data Protection Commisioner (I think it's called) and they'll be all over it like a rash.