Moorcroft are a complete joke. The current, and last firm I worked for use them to recover debt's, and even if our customers pay the debt to us instead of moorcroft, they still persue it.
Don't worry about the balance increasing, or any credit reference agency's being involved, they are one of the companies that use all the threats in the world, but no action is ever taken.
If you don't get a reply to your letter within 10 working days contact the ombudsman directly. Upon doing this you will receive a written reply with a full explanation from the AA within 10 working days, as this is a stipulation set by the ombudsman.
Good luck with it mate.