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If you want to change your return date as others have said you need to give 8 weeks' notice. If it's less than 8 weeks until your planned return date, just give notice as soon as possible, and I would expect your employer to agree it if possible.
Im kind of in a similar situation, mat leave ends in Jan but I wanted to take my annual leave to extend it so the baby is a bit older and we are both more settled with childcare - however, I think a meeting that I will be attending will be to discuss me not taking that leave now, how did you get on/are getting on with it?
I think they are allowed to ask you to go back if they need you in etc (thats what Im seeing in the legal guidance anyway...)
Including your email, you've put it in writing twice so I think it would definitely count as notice to return.
You should give 8 weeks notice, but my place if work wouldn't hold me to it if it was for a genuine reason like yours. In fact, even if I just wanted to take longer they'd be fine. All you need to do is ring them/email them as early as possible. January is still 5 weeks away, I can't imagine they can insist you come back if you don't feel your baby is ready to be left. But get in touch as soon as you can, don't leave it much longer.
I have been on mat leave a few months and was planning to go back in January. Filled in a form before going off that asked for planned return date and said January. Was informally asked at the end of October and said January.
However, weaning is not going well and I think I would rather return in April when baby will be a bit older. Can I change my mind? How? Am terrified I have left it too late...