If you're planning on handing in your notice immediately you get back then they may not require you to work your notice period. In that case, if your notice is 4 weeks you may be able to use that 4 weeks salary against it with any unused hols, which you should have plenty of.
I'm only back from mat leave since the summer so I know all about how hard it is to concentrate on things other than your new first baby. BUt I have to say, there is absolutely no way that I wouldn't notice an extra €1200 in my account every month. Especially since I took all my unpaid so we had kept an eye on our finances so that we could afford that luxury.
As for the company getting shafted, I don't agree there, they could easily have a clawback clause to deal with people not returning. If I left within 6 mths of returning, I had to repay all the topups, if between 6mths and 1 year, then half the topups. I had to sign an agreemnet before they'd pay me anything. This is an incredibly simple, fair system and a company who doesn't implement something similar is implying that they don't mind if people don't return. Not everyone who doesn't return plans it that way, sometimes people have no choice.