Since returning from maternity I have gone part time (20 hours) however my employer is continuing to overload me with a workload I would struggle with full time. I am continually doing at least 30 hours and have been unable to take any holiday for the last 6 months (I even did 25 hours Christmas week). When I did have some booked I ended up working through it.
I have raised the issue and was told if I can't cope with the workload they will find somebody else who can. My workload has not been reduced at all since returning from maternity, just my salary by 50%.
Raise a grievance. There is a statutory grievance procedure that can be followed. Your employer agreed to your change of hours, they didn't have to, but having done so they should be making it work. It sounds like they are trying to force you into resigning, which is very silly as that is effectively a dismissal and you could take them to employment tribunal.