I put in a formal flexible working request for when I return from maternity leave and this was eventually agreed, but then I was told over the phone that the agreement was temporary and would be reviewed and they are now trying to take it away from me. Another colleague has, however, been offered part-time work. I did say at the time that I could not work full time, but can they say I have accepted the temporary arrangement in the meantime?
Have you had anything in writing or an amendment to your contract? Have you returned to work since being on maternity leave? If they are making any changes to your contract then they will need to write to you to let you know, especially if it was agreed. The change will most likely have an impact on your wages and your holiday so you should have this change documented.
Also you say that they offered the other team member part time. Was this a job share? It might be worth speaking to the other lady to see what she works, and if her request was approved.
Legally, once you have put in a flexible working request you must have an outcome within three months, the reason for rejecting the request must be one of the eight legally permitted ones and the reasons must be factually correct.
Once a change has been agreed and made it should be a change/ amend to contract and the change should be permanent. The contract can then only be amended with mutual agreement or if another flexible working request is put in and agreed.