You asked if interest can be charged and the act I quoted (Late payment of commercial debts (interest) act 1998) as the one that can be used, I am sorry you found it difficult to understand. You must of course be liable, but if you are and do not pay they can charge £40 and 8 % over base rate up to the time of payment. I have used it a number of times and most people have paid, the threat of using it, which you are experiencing, is usually enough. But I repeat you must be liable in the first place, if you believe you are not get the advice of a solicitor experienced in the area of law you require, the law society may be able to help 020 7242 1222.