Basically you initially need to write to the creditor by recorded delivery, stating the reason(s) why you consider the PPI was mis-sold, requesting the return of your premiums and any added interest or fees.
Inform them you consider the imposition of this punitive fine as unfair treatment in your case, that the unfair placing of this default notice has resulted only as a direct consequence of the mis-sold PPI being applied to your account. Explain that you consider the default would never have occurred had the PPI not been applied to your account in the first instance... request the immediate removal of this unfair default that has been applied on your credit file for all the wrong reasons.
If you cannot resolve this matter after Eight weeks, you are then entitled to ask the FOS below... to look into your complaint and overturn the creditors decision, should the FOS decide to the contrary in your favour... referrals to the FOS cost the organistion under examination a case fee whether you are successful or not, you may remind them you intend to take your case to the FOS.... if your letter fails to reach a satisfactory resolution.!
http://www.financial-ombudsman.org.uk/consumer/complaints.htm
I enclose other relevant sources from the FOS about PPI, for your information.
http://www.financial-ombudsman.org.uk/publications/technical_notes/ppi.html