Let me get this straight.
Your organisation operated a return to work bonus after maternity leave. You were paid this after DC1?
At some point they decided that they wouldn't pay the bonus anymore and have stopped it.
Firstly, sorry, but it's not relevant that they've continued with other bonuses, since you presumably participate in those either fully or pro rata (depending on the structure of the bonus) whilst you are off.
In terms of the return bonus, was this set out in writing anywhere? If not, did they do it for everyone? If it was written down, have they changed the written policy? If it was written down, what was the exact wording and did it say that it was discretionary and could be withdrawn? Have other people returned and not been paid it? How big is your organisation?
You say you think you should have been told "in advance" about the bonus. Do you mean before they changed the policy? Because you are being told in advance of the return, IYSWIM.
Sorry, lots of questions...