Did I suffer maternity discrimination?

[duckme]

I'm looking for someone/people to either tell me that I'm being over sensitive and need to get over myself or that I am correct and I have suffered some sort of low level discrimination at work.
I had my third child in 2017. I've been employed at my current place of work since 2014 and took a promotion (applied for through usual channels and interview for the role) in 2015.
I left for maternity leave in October 2017 and returned to work in July 2018.
An advert for a temporary position to cover my role was placed and was filled. I had nothing to do with this process-at my request.
I notified my intention to return to work (date of return and request to reduce to part time) in May.
At some point during my leave, a colleague was promoted, including a pay rise to bring her in line with my level of pay. Her position now included the financial aspect of my pre-maternity leave role. She didn't apply through normal channels and was not interviewed for the position. I was not informed about this or asked if I would be interested in this new role or told my role would have changed when I returned.
It was then decided, presumably because I had requested part time hours, to keep on my temporary maternity cover permanently. My reduced hours didn't require an extra member of staff-this is supported by my colleague who said the same to my line manager.
I returned to work to find my role had basically been allocated to other members of staff (mainly the colleague who had been promoted whilst I was away) meaning I effectively had no duties left.
I loved my job, I was dedicated, reliable and efficient. The years since I returned from maternity leave have had a hugely detrimental impact on my mental health. My confidence has been affected massively as I spend every day scrabbling for things to do. I have mentioned my concerns to my line manager and my colleagues but nothing has changed.

I am looking for another job but I also feel as though I shouldn't have to be in this position just because I had a baby.

I feel as though I have been unfairly treated whilst on maternity leave and since returning to work. But, and thank you if you've managed to stay with me so far, I am definitely happy to be told that I am in the wrong and my employer acted in accordance with the law.
Any and all advice will be gratefully received.

You can contact Pregnant then Screwed for free advice x

Your right to return at that point was to a role of the same terms and conditions, not the exact same role you left. They have honoured this. So ultimately, I think that you need to decide whether to stay or go. And going sounds better for you.

As others have said you returned after additional maternity leave so your job content can change as long as terms and conditions remain the same and the role is within your skills.

Add to that you've been in this role since 2018. That's coming up to 4 years. The fact you've continued to do the new job for that time shows you've basically accepted the change in role.

By all means look for a new job but I'm not sure you can claim anything.

Yes, as others have said, if you take longer than 6 months mat leave then they can change your job role unilaterally as long as the Ts&Cs remain the same

Thank you all for your replies. They're very helpful.
Just a final question, if my colleague was promoted during the first 6 months of my leave, would that make a difference?

@duckme

Thank you all for your replies. They're very helpful. Just a final question, if my colleague was promoted during the first 6 months of my leave, would that make a difference?

Not likely, especially if they knew you were planning to be off longer.

Also it's been 4 years. I think you're out of time to complain really.

What has sparked you wanting to complain now?

Agree with other posters - what has happened now that you want to bring this up?

I work in a school and we went through some quite substantial process and system changes since I've returned after leave. I was quite involved in this process and it did occupy a fair amount of time in the couple of months it to to implement. I then offered to help out at another trust school as I had completed the work at my school and had gone back to having nothing to do.
Then covid hit and everything was sort of up in the air again.
It's only really been the last year which has been 'normal'.
I totally understand that it's been so long since I returned that it's too late to do anything about it-though I have mentioned it to my line manager and the headteacher.