Chris
I'm afraid I cannot guarantee how the DWP will view this but there is a clear difference between voluntary work and Permitted Work.
I'm afraid it's a decision you will have to make for yourself based on your conditions and the activities that you have to do at the hospital.
Be aware you need to tell the DWP if you are starting any voluntary work.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems