Debt collection complaint

Hi. Can anybody help with this problem I've got with BT...

After much incompetent messing around with my phone account by BT, I got a letter from a firm called Credit Solutions Ltd. (CSL), on behalf of BT. I was somewhat surprised, as I had been paying my bill by Direct Debit. I checked with my bank and found that the DD was still valid, but that BT hadn't taken any money. I told CSL this and sent them a copy of the printout my bank had given them.

I expected that this would be the end of the matter but CSL sent me another letter, this time threatening to go to court and charge me about three times the amount of the bill in costs. I went back to my bank and this time I got another printout, signed and stamped by the bank to say that there was no reason why BT couldn't take their money if they wanted it.

I phoned BT to ask why they were still treating this as a debt, and they told me that the account had been cleared (I hadn't made any payment) . So I phoned CSL to tell them this. They said they would have to check with BT and I have heard no more from them since. (This was early in January).

Today, I have just received a letter from a different company, demanding payment of the amount originally outstanding on the phone account, plus CSL's charges.

I have no fears about going to court, as I have the letter from the bank showing that BT could have taken the money at any time if they had wanted to (and still can). What I want to do is to make sure that they go away, and stay away, without damaging my credit rating, as I have done nothing wrong.

I spoke to Consumer Direct about this and they gave me details of some organisations to contact, but these were all to do with complaints against bailiffs.

Can anybody advise me how to pursue a complaint against BT for unfair debt collection practices?