The Office of Fair Trading have a list of guidelines by which debt collection agencies must abide, http://www.oft.gov.uk/shared_oft/business_leaflets/consumer_credit/oft664.pdf
This document contains a list of practices which the OFT consider unfair, including:
2.4(f) "pursuing third parties for payment when they are not liable"
2.6(h) "ignoring and/or disregarding claims that debts have been settled or are disputed and continuing to make unjustified demands for payment"
2.8(a) "sending demands for payment to an individual when it is uncertain that they are the debtor in question, for example, threatening debt recovery action to 'the occupier' or sending a payment demand to all people sharing the same name/date of birth as a debtor in the hope that contact with the correct debtor will be made."
2.8(i) "failing to investigate and/or provide details as appropriate, when a debt is queried or disputed, possibly resulting in debtors being wrongly pursued"
2.8(k) "not ceasing collection activity whilst investigating a reasonably queried or disputed debt."
If you believe any of the guidelines listed above, or any other of the guidelines listed in the document, may have been breached by the debt collection agency in question with regards to their dealings with you, then a complaints form which may be used to report any breaches to the OFT is available here, http://www.oft.gov.uk/shared_oft/business_leaflets/consumer_credit/DebtCollectionComplaintForm.DOC
Further information is also available here, http://www.oft.gov.uk/advice_and_resources/resource_base/legal/cca/debt-collection