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Blooming Marvellous
Whilst I agree that these sort of firms with the PPI claims and suchlike are ***** (you sign it, you deal with it in my opinion) how would it be giving you a bad credit rating? Surely the debt would simply be marked as "Settled" and the credit file wouldn't say whether this was a cash payment or a court case so the overall effect would be a better credit rating (assuming you're paying your other bills on time)
You take a gamble that the company will not challenge your appeal, if they do you could be liable for costs and this will be marked. The easiest way for it to be marked is what someone mentioned earlier within any contracts they can close all/any accounts s long as they give you 30 days notice. This will then get your next application for credit refused which in turns starts a downward spiral.