@MyBossIsAPillock Firstly OP, treatment for cancer is bad enough and you ARE still being treated for cancer, you haven't had cancer in the past tense, you are still being treated now. You really don't need his attitude and from the sounds of it, he is not "managing" at all and he is in danger of losing his job.
I have to say that I can understand he may have legitimate staffing issues but that is what he is paid for as a manager, to manage, not terrorise seriously ill people into potentially putting their life on the line.
This isn't annual leave. It isn't even a request for time off. It's medically necessary absence for cancer treatment. In the UK, someone having radiotherapy for breast cancer would normally be treated as being on sick leave, not holiday. The manager's comments about it being half-term, other people booking leave, or you "barely being in this year" are completely irrelevant if the absence is because of cancer treatment.
What makes it even worse is that he clearly knows why you've been absent. Saying you are "taking the piss" because you've had time off for breast cancer shows either astonishing ignorance or a complete lack of empathy. From a practical employment perspective, he's potentially creating a huge problem for himself and the employer: Cancer is automatically treated as a disability under the Equality Act 2010. Employers have a duty not to discriminate because of that disability. Absences connected with cancer treatment are generally handled very differently from ordinary sickness absence. Threatening disciplinary action because someone is attending radiotherapy could easily end up with HR taking a very dim view of the manager's behaviour.
The fact that you say HR and senior management have been excellent throughout is reassuring. If you've accurately reported the conversation, I suspect HR would shut that nonsense down extremely quickly. Most HR departments become very alert when they hear words like "cancer", "radiotherapy", "disciplinary", and "taking the piss" in the same sentence.
What also struck me is that you did exactly what a reasonable employee should do. You gave six weeks' notice of a known medical treatment schedule. You aren't ringing in on the morning saying you fancied a fortnight off. You are informing them of a hospital treatment plan. Its not like its an elective treatment - sorry boss having a fortnight off for a facelift! This is survival!
Honestly, I suspect many Mumsnet readers will be reacting less to the employment law angle and more to the sheer lack of basic human decency. Even if there were no legal implications at all, who speaks to someone having cancer treatment like that?
If the post is accurate, I think you should forward the entire chain of events to HR (hoping its in writing)and then concentrate on getting through your radiotherapy. You have far bigger battles to fight than a manager who appears to have misunderstood the difference between annual leave and cancer treatment.