I'd also say that further correspondence beyond a response to the letter will be regarded as harrassment and proceedings for the same may follow.In short it's not enforecable and you should tell them to fuck off.
McNimtm
McDonalds was a consolation after Harry Potter.
What! No dummy spitting tantrum at being branded a Mc?What is the world coming to?
I'm going to send out 1000 letters demanding £50 for some made up, unenforceable transgression and see how many pay up just to get rid of the bother.