This is a simple matter of automated correspondence. I have had similar (unsurprising). In the absence of you knowing when the last sorn, if indeed it exists, was issued and therefore expires, the onus is upon them to send you a timely reminder to either tax or sorn the car. Without this you cannot be expected to know what is due or when. You need to send a letter in response explaining the change of keeper and quote "their absence of issue of timely reminder" You should then get a non apology letter saying the matter has been dropped subject to you registering the car SORN.
The department which deals is not in Swansea, its in Nottingham or Northampton; use
http://www.saynoto0870.com to get the direct person to person telephone number.