Hi rocky,
PIP has deliberately made more difficult to get than DLA, the criteria for the 2 benefits are very different.
The assessor will decide whether they believe they have enough information to make a recommendation to DWP without seeing you. Otherwise you will be required to attend a face2face assessment.
The best chance of getting a favourable decision without a face2face appointment is to explain your problems, as they relate to the PIP descriptors, in detail and provide as much supporting documentation as you can.
Have you actually received a letter 'inviting' you to apply for PIP?
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
