Peter
Just notify the DWP the day before you start work that you wish to close your claim and that you are doing so because you are starting work. I would do it in writing but also phone the DWP office dealing with your claim.
There is no specific requirement for you to tell the PIP department that you are starting work, it may disadvantage you if you do not tell them and they decide that your being able to work is due to an improvement in your conditions.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems