Pegleg
Welcome to the forum, you might want to have a look at the following FAQ which explain where everything is
Welcome to Benefits and Work My advice would be to do exactly as you have suggested in your post.
The DWP letter needs to be addressed to the office dealing with your claim, see the latest DWP letter you have for this, mark it for the attention of the "Decision Maker"
Assessors are told they need to accept new evidence on the day, to read it and include it in the claimants file.
Gordon