Kat
In terms of the assessment process, take a copy with you and ask the assessor to read it and include it in your file when it is returned to the DPW.
As to the missing letter, give it a couple of days and then call again, I've seen some posts that there are considerable delays in receiving letters, etc. due to delays in the scanning system at Wolverhampton, so it may have been received but not yet scanned.
If it is still missing then consider a formal complaint while you are on the phone to them.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems