[Margaret Pilkington]

There is a template letter on the first link that Accyjay gave. It is a good place to start.
It lets them know that you will not put up with harassment unless they provide proof that you do actually owe the money....it also quotes the Consumer Credit Act.(You can google for more details on this act of law)

Have a look at the link Rishtongirl.....don't let them scare you into putting money(which you may not owe) into their pockets.