Hi grw
If you think you are going to struggle to get the form back on time - ring the DWP now and ask for an extension. Normally they will give you an additional two weeks.
Have a look at the Guide to PIP Claims and Reviews and read the advice there. The information you need is from page 128 onwards
benefitsandwork.co.uk/guides-for-claimants/pip. Even if you are going to tick 'No change' we still advise that you write something. It explains why - on those pages. Once you've had a look if you have any further questions - come back and ask and we'll try and help.
BIS