You need to inform the DWP office dealing with your ESA if you start any PW.
There is no specific requirement for you to contact the PIP office but it may disadvantage you if you do not and the DWP decide that the work that you are doing is the result of an improvement in your conditions.
I would expect the DWP to pass on to any assessor the fact that you are working and this may influence how they look at your limitations.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems