Newbie
You are within the Permitted Work rules of less than 16 hours a week and less than £131.50 a week.
You need to contact the DWP about the work.
Have you considered whether your doing this work is at odds with the reasons that you were awarded your ESA? Could this be seen as being the result of an improvement in your conditions?
There is no specific requirement for you to notify the PIP department but if you fail to do so and they view your working as being the result of a Change then it could disadvantage you if you have not.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems