Can my employer renege on a flexible working agreement?

I am currently on maternity leave, due back to work in the summer. I work for the NHS as a PA. I met with my line manager and one of our senior managers in December and agreed a formal flexible working arrangement for when I return. I have not requested to reduce my hours, just to work them differently. The agreement was put in writing and I went away and made my childcare arrangements. My mother advised her manager that she will be reducing her hours to care for my son, the hours she is giving up are going to be shared amongst her colleagues. Yesterday I received a call from my line manager informing me that our group manager, who is above the senior manager who authorised my request (we have a lot of managers), isn’t happy about the agreement and will be calling me next week to ask me to go in a meeting with her. I want to know where I stand if she tries to retract the agreement, which I suspect she is going to do. As I say I have it in writing so can she do this? There have been no changes to the service since it was agreed. There was also no mention of the agreement being temporary so I assume this should now be a permanent change to my contract? This is now causing me a great deal of stress and it is not just me who it will impact on – there is also my mother and her colleagues to consider now.