I thought you had a right to correct entries on your credit rating - not expunge but put in your side.
Anyway Here are the 8 data principles that the credit companies have to follow:
http://www.belb.org.uk/downloads/foi_data_principles.pdf
and you will see that 4. is accurate and where necessary up to date
so clearly you have a claim on that.
Start off with the data controller at the debt collectors
and THEn refer to the Information Commissioner.
by experience the IC is great on freedom of info
and pretty crap at data protection
but that sholdnt stop you from going down this road.