Interest and charges can only be added to your debt if they’re included in the credit agreement you signed when you first took out the debt.
But the following could be classed as unfair, and if a creditor does any of these you should consider making a complaint:
Increasing the rate of interest because you’ve missed payments
Continuing to add interest and charges if you’re in financial difficulties, making the debt increase
Adding charges for actions the creditor hasn’t done, for example adding ‘legal costs’ or ‘solicitors fees’ to a debt when no legal action has been taken
Adding charges which are more than the actual cost to the creditor for collecting a debt. For example, adding a charge of £100 for sending a letter would be unfair because that’s a lot more than the cost to the creditor