Your employer is ignorant of the law. 

I suggest you ask your employers to familiarise themselves with the terms of the Equality Act 2010.  More here: https://www.macmillan.org.uk/information-and-support/organising/work-and-cancer/if-youre-an-employer/legislation-about-work-and-cancer.html

I think allowing you to work in the morning comes under the term of "reasonable adjustment".  And requesting you to work in the afternoons with the threat of sacking is a clear breach of the law, since you are being harrassed, discriminated against, and victimised. 

I don't suppose you're in a union, which is a great pity since a union would help you stand up for your rights and face down this bullying tactic. 

Thank you for your reply. I work in the morning, but they suddenly want to change to do afternoon. I might join a union. I'm so tempted to tell them to stick it. I just want them to understand.

Just to add to the great advice you have had is maybe speak tot he Citizens Advice they have people who specialise in this and do Tribunal work which is another way you could go as it would amount to constructive dismissal by forcing you to work outside a reasonable adjustment.

Thank you for replying. I've been to citizens advice and I was told my employer can't make me, even though it's in my contract. Had a meeting with HR at work and was told I've got to do it or reduce my hours and then get someone else in or leave. I don't want to go down the legal route, but it might have to. 

Hi there again ...

If you ring McMillan and tell them what's happened they have a legal adviser that just may help  .. I'm sure they CAN'T do that, when someone's going through cancer ... 

I know rilleyroo went through Simerla and Dave knows quite a lot about how and where to get advice ..hang on in there ... keep a recorded of everything .. and what they said and when ... I'll ask them to pop by .. chrissie. . Don't you give up .. or well all go round and sort them right out lol ... chrissie x

Hi there again ...

If you ring McMillan and tell them what's happened they have a legal adviser that just may help  .. I'm sure they CAN'T do that, when someone's going through cancer ... 

I know rilleyroo went through Simerla and Dave knows quite a lot about how and where to get advice ..hang on in there ... keep a recorded of everything .. and what they said and when ... I'll ask them to pop by .. chrissie. . Don't you give up .. or well all go round and sort them right out lol ... chrissie x