Given new role, but then told I have to return to the old one

I have worked full-time with my employer since 2010. After maternity leave I requested flexible working to change my hours, but was unsuccessful and told there were no alternative positions. A week before I was due to go back, a colleague in the department who worked later hours left, freeing up that position. I immediately contacted HR and requested I return in that role as the hours fitted with my requirements. This was granted and I have been performing that role for the last six months. No new contract was issued, nor did I sign anything to this effect. The only evidence I have of this situation is in the form of emails between HR offering those hours and me accepting. But does that not constitute some form of contract in itself? Since May my manager has been pressurising me to return to the earlier hours. I have been given seven weeks’ notice. My manager claims this is due to a “restructure” in the department whereby he wants everyone in the team to be able to do everyone’s jobs and have them on an alternating rota from week to week, although a colleague who was granted flexible working is exempt from this. I appealed this decision. I have been told that if I do not agree to the change I will be in breach of contract. They have offered me another role, which is effectively a demotion and £4,500 less per annum. It feels like the punishment for having a baby is being demoted and given a pay cut. What can I do?