I've been getting these letters on a regular basis for over 2 years now and I have a TV licence.
The problem arises from the way my address is shown - My licence shows my house number as 10d, but the threatening letters are addressed to 10 (1/2) which is the same flat.
In the early stages, I phoned, wrote and emailed to try to clear up the misunderstanding, all to no avail. They are a bunch of administrative tosspots.
My favourite bit is at the end of the letter when they say if you've recently bought a licence, to accept their apologies and ignore the letter.
I do. I will.