Hi Holly,
You could ring the PIP enquiry line and say that you want to correct your evidence based on the assumption that you misunderstood. I can't see any reason why they wouldn't type it up on the computer with additional notes (although I don't believe it's down to them to advise on certain information, you could always ask). Failing that, you could just add it at a later stage during an MR or tribunal hearing, should it go that far. A lot of PIP questions are primarily concerned with "how things affect you on a daily basis".
Hope this helps.
Regards, Chris.
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
