Hi Mum2Monkey,
I had a similar situation ...
I work in the city, took almost 6 months maternity leave off (1st baby), then went back to work full-time (flexible working was not an option). On return, one of the best parts of my job had been given to someone else. I did not think it was discrimination (as business moves on...), however i could not/cannot stop being resentful about it. Changing company was one of the first things i thought about
..
I decided to get on with it in the short run (the job is still interesting, well paid, good team, i leave at 6 every day so i make it to "bath time"), have another baby and 2nd maternity leave, return to work and then look for a new job.
So my advice would be to make the situation work for you the best way you can...I know it feels frustrating, but don't act out of frustration, act with a clear head once you've decided what your priorities are.
I personally didn't consider going down the discrimination case route, I judged that it would probably take me nowhere, so it would be a wasted effort.
I do think though that you should try to leaving work earlier and see what happens. I am leaving about 1.5 hours earlier than before i had DS, and it somehow seems it is expected...
Good luck with it all!