R
PIP is not a work relates benefit so there is no specific requirement for you to tell the DWP that you are starting work but your post suggests that you are concerned whether the work will be at odds with your Mobility award (at the least), under these circumstances it may be to your disadvantage if you do not tell the DWP!
If you do then you will almost certainly be reassessed, a new claim form and a face to face assessment.
You should expect your existing award to be reassessed one year in advance of it ending.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems