Many thanks for your patience. Once you return from maternity leave (assuming you took more than 6 months off) you do not have the automatic right to return to your old job. According to Reg. 18 of The Maternity and Parental Leave Regulations 1999, “if it is not reasonably practicable for the employer to permit you to return to the original job, they must allow you to return to another job which is both suitable and appropriate for you to do in the circumstances.”
This is the position if you do nothing and just try to come back as normal. In your case you made a request to go part time, so this is a flexible working request basically made by you. The changes you apply for would be permanent, unless otherwise agreed between you and the employer. Here you were told you are going to trial these changes for 3 months. What is important is whether this trial was intended for you or for them, or for both. Sometimes the employer will place a trial for their own benefit to see if this arrangement works and if it does, they can automatically agree the changes, in which case they would become permanent. If they gave the trial for you to determine if this was a suitable change, then they should still seek your opinion on whether you considered these changes suitable before they went ahead and made them permanent. So it all very much depends on what the purpose of the trial was and, more importantly, for whose benefit it was.
In any event, their failure to budge on this means you only have a couple of options on challenging them - one is to raise a formal grievance with them to get this looked at formally by management. If that fails, in reality all you have left is to resign and consider a constructive dismissal claim against them.
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