Before I started on MIND reception, on my application form I explained my situation and made my limits really clear, whilst also pointing out that my health is improving and it is likely that I will be able to take on more work in the future.
I've played it safe and for now, have only offered to do an amount of work that I'm 99% sure I can definitely do, so as to be fair on everyone.
I explained the situation further when I had an interview, pointing out that I'm well enough now so that I won't be ringing in sick with my ME at the last minute - to reassure them that I will be reliable.