Hi Simon
Thanks for your post and your openness.
I don't know what the CAA's stance is, but it might be worth investigating whether you can still fly P/UT if you feel up to it of course, (as I read it you are part way through your ppl(h)?).
My Brother-In Law had a very similar condition, and asked if he could ride his motorbike between Chemo sessions. He was told that if he felt up to it there was no reason why not. (for the record his chemo went well and he has been clear for 5 years now.)
Best of luck.
Cheers Dan