Basically your last post says :-
The original owners - your colleagues of the last 16 years - either lied or "forgot " to tell the new owners the original policy. They even "admit that the new owners should have been told about this old agreement".
So, your statement in the OP, that you are "repulsed" and that "the company are being very underhand" is difficult to understand. By "the company" do you mean the new bosses who seem to have acted in good faith? Surely not.
Based only what you have said it seems that your colleagues,the old bosses, have a moral responsibility to fork out the extra maternity pay from their own resources. Their attitude seems to be "yeah,well, we took their money without telling them the whole truth and now the suckers have to pay out for your maternity leave -tough on them and maybe tough on you having to fight for it ". Nice people !
Some years ago I was a director in a large travel company that purchased several smaller travel agencies, the owners of which were retained as managers of those shops.
The purchase agreements contained provisions that basically said if there are any falsehoods, omissions etc etc then the old owners had to pay up.
Obviously,it was more complicated than that but a failure to disclose salient facts would, i thought, have made your old bosses liable.
However, not knowing what type of contract your new bosses had with your less-than-open old bosses it's difficult to say whether the new bosses could succesfully sue for the extra maternity benefit, to which legally I still think you are entitled.