I'm glad that you have someone to go with you, I would never have coped with the assessment alone
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Personally I would take copies of medical evidence already provided so that you can refer to it and just in case they have 'lost' it. Is there any further evidence that you haven't sent yet that might help? If so take that to the assessment.
Thought of a few more things;
Try and write some answers down too, chances are you'll be quite stressed and might forget certain things that you need to say.
Are there any adjustments you need? I didn't realise that you could ask for a different less claustrophobic room which would have helped loads.
I'd also say be suspicious of everything they ask, even chatty questions. An example from my assessment was that I was asked if I like to read and when I said yes she said that I can obviously concentrate then so I had to explain that I cannot do so on my worst days. They'll also ask how you got to the assessment, make sure that they know how stressful it was and how anxious you've been in the run up, this was used towards my 'Planning and following journeys' points.
You won't have to go through a physical examination if it's for MH and remember that you don't have to show them SH scars so try not to worry about that side of things. Have you been through the questions with your PA to prepare for the assessment? It might help to think about why you need PIP- consider things like needing a PA to help motivate you and people to help you go to strange placesetc. Or when you're high and you might spend too much and need someone to help you that sort of thing.