Thank you @arpie,
I have an oncologist appointment next week, I have a list of side effects to tell him. He did say if they don't suit I could try another one, but he also said (surprisingly) that my cancer was "so small" and with no lymph nodes involved I could choose to stop taking them! Whereas my GP when I mentioned not taking any said it was "not an option".
I hadn't thought of medicinal cannabis for pain, but is it in your system if stopped by the police for a drug test? I don't want to loose the ability to drive, even if it would help with sleeping. :|