Re: Comebacks...
Is there no way that you could accommodate your employer, as they facilitate you with your request in the past. If you are unable to accommodate your employer, then speak to a union rep if you have one or else contact someone listed by the other contributors.
IMHO
Check your contract to see what that says
As you are at least a year in your new part time role it is now custom & practice.
I do not believe that they can dismiss you without offering you a redundancy package you could accept.
I can understand to a degree your employers predicament, but it was their final decision to let you go part time. Through not fault of your own they are now going to dismiss you because of a decision they made in the past.
If they dismiss you and replace you with somebody this is a case of constructive dismissal.