If you are not being paid, then you do not need to inform the DWP. But do think carefully about how your new role could be viewed when your ESA is next reviewed. The ability to do a voluntary job could be seen as evidence that you can do a paid job.
There is a gentleman who does some amazing voluntary work where I am based, but periodically we get a call from the DWP asking us what he does, how capable he is of working without support etc etc. I am sure you get the picture.
Hope it all goes well for you, Catherine
Nothing on this board constitutes legal advice - always consult a professional about specific problems
