If my pip assessment isn't successful can I send in more evidence at that point (would be dated from before the claim so not new) or would I need to start a new claim?
If you have just had an assessment then you need to contact DWP and ask for a copy of the assessors report (PA4). There is a window for you to write to the Decision Maker before you get your decision letter, to correct anything in the report that you disagree with, but you will need to be quick, you can also enclose additional evidence to support your claim.
You can enclose additional evidence at the Mandatory Reconsideration stage and at the Appeal stage.
You do not need to start a new claim to send in additional evidence.
Gary
Nothing on this board constitutes legal advice - always consult a professional about specific problems