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Thanks ppl, I have faxed them another letter, with copy of the cheque, clearly showing the 'alteration', and threatened them with legal action. A friend suggested that as they are a Debt Collection Agency I send them three faxes in a row saying 'I have been round to your property, and you did not answer the door, even though I saw your curtain twitch' :-D ...on a more serious note, I'm also reporting them to Abbey National, the company who I originally had the debt with.