Hi Tom and Welcome to the forum, you might want to have a look at the following FAQ which explains where everything is
Welcome to Benefits and Work It depends where the letter has come from. You should not need an assessment for PIP if you recently had a decision regarding this. Any letter about this would be from ATOS or Capita.
It might be that the assessment is relating to your ESA, in which case the letter would have come from Maximus/HAAS. Our ESA guide explains what to expect
ESA claims and appeals