My friend was having trouble with her CTC (like many others). Well she informed them of a change to her job so they adjusted her payment. She contacted them by phone and letter to check they weren't over paying her but they insisted she was getting the correct payment. After almost a year, they sent her a letter to say she has been over paid and owed them X amount of money.
After complaining to them by letter, she received a reply stating that as it was their fault, she does not have to repay them anything.
Hope others having problems with them can do the same if they are in that situation.