Hi lee
It could be for a number of reasons, we do not have all the information, We would advise you to seek help from your local Welfare Rights Specialist; you can find your local Welfare Rights Organisation to help you with advice by clicking on the link below and putting in your post code, it will then come up with a MAP with a list of agencies in your area
advicelocal.uk One of the questions I would ask you, were you in receipt of CB ESA? if you were then that could be the reason why you received the letter, but I am only guessing.
Gary