Charlie
There is a guide to PW in the ESA area but in summary; you can work for less than 16 hours a week and earn up to £125.50 a week after deductions.
You need to inform the DWP that you want to do the PW in advance and obviously, the work must not conflict with the reasons that you have an ESA award.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems