A dispute and unfair banking charges do not affect your credit score. CCJ's and the like do. I was bad and got loads of debt when I was a young squaddie, debt agencies after me etc and it didnt make much difference as it never went to court or recieved a CCJ.
You can get your credit score through
http://www.experian.co.uk for free. I signed up got what I needed then cancelled it free of charge and found out my credit score was 971 out of 1000. 1000 being the best!
I was told by my loan company I had a bad score and could only get a loan at 19.9% apr. Once I found out how good my credit was I went else where and got a tesco loan at 6.9% apr, paid the lloyds loan off then smashed them for banking charges.
Just because some one says you have bad credit does not mean you do. People doing finance want you to have a higher apr as they get a bonus for it!
Find out for your self so you can tell them to sod off! You might even be better off with a bank loan than finance. Check these options first mate,