A call to Macmillan would be useful I think. They are very clued up on this sort of thing.
How can your employer say that your illnesses are not cancer-related! How can they prove this?
It is the person who is covered by the disability discrimination act, not the ailments. As far as I am aware, having had a cancer diagnosis you are covered.
Have you thought about taking a union representative with you to the OH appointment.
Firstly well done on getting through treatment and back to work and fitness. This happened to me when I returned to work after treatment in 2011. I was still having Herceptin when I went back to work and there was never any problem with taking time to go for this. But I found like you for around 18 months I seemed to catch every sore throat and bug known to man. I don't think it helped working in an office where bugs were and travelling to and from work on the train with people coughing and sneezing.
I was told I had to see the firm's OH doctor when she came in as I had over stepped the 10 day sick policy. She was excellent, told me that of course I would get bugs etc as my immune system was basically "shot at" with all the chemo etc. She did a very stern report to HR telling them this in no uncertain terms, and also suggesting that they needed to improve their treatment of cancer survivors.
Needless to say I had no more problems with HR.
As time goes on you will find you get less and less bugs etc. I am now 4 years down the line and this year I haven't had one sick day from work, so you will get there.
Remember, OH is there for YOU, not the company. They are there to make sure that the company don't overstep the mark with you. Go in there all guns blazing!!
I'm sorry you are having problems with illnesses and work. The Macmillan helpline and website have lots of information about work related issues. I've attached a link:
I hope this helps.
Very best wishes