Ok well they have definitely not treated you properly!
Essentially they have not kept your original (pre-mat leave) job open for you and replaced you permanently with this other person. This is illegal.
However, if your pre-mat leave job really cannot be done flexibly and your business can demonstrate this without a doubt, they might be within their rights. But the way they have done it is inappropriate.
It is also illegal to make false representations about your employment and if your manager told you it could be done flexibly which encouraged you to accept it, and then he says it can't actually be done flexibly, again this is illegal.
I would speak to HR about all of this. Hopefully they have a clue!!
I personally wouldn't rescind my resignation as unfortunately your working relationships will be tainted by this, as crap as that is. I would take the new job and make a complaint either through FWA or a discrimination claim. It depends what avenue you go down (I.e the false representations thing would be via FWA, not EEO).