Received a letter regarding a debt which ive had a...

Received a letter regarding a debt which ive had a payment plan in place for 2yrs without any breaks. I blame HMRC who keep sending stupid letters threatening to send bailiffs round and everytime I call them they apologise and say it was a mistake. They still sent my personal details to a debt agency Advantis whereupon they apologised again. The countries gone to the dogs.