One option is to write with full details (including the time and method of cancellation and mentioning the "distance selling regulations" - assuming this is relevant). In the letter you could state that, unless the matter is settled by the end of November, you will be seeking the money through other means. Because of the current postal uncertainties, you could send the letter by recorded delivery. After your letter has arrived, you could also follow it up with a phone call.
If the matter isn't settled by the date that you give, you could contact your bank and ask them to start their procedure for disputed payments. You can give them a copy of your letter. If that doesn't work, you could use the following site to make a legal claim.
http://www.moneyclaim.gov.uk/