If you don't want to spend the $$ to call London, call the Marriott customer service line & explain the situation & say you'll fax them the confirmation of the cancel. They should take care of it. If not, then call the LHR Marriott & say the same.

BTW - I'm assuming you were on a refundable rate. If you weren't, then even though you canceled in advance you'd still get charged. If you were, then challenge it.

PS - Welcome to FT!