You see, I did read around, albeit subsequently, and the reading I've come across states yes, it is a right of landlords to take rubbish tax (and yes, I was aware of that - we paid it monthly at the last place) but that not all landlords choose to exercise that right. Now given that, it would be reasonable to assume, in the absence of any other information, that the monthly charges we were already paying included the tax. With hindsight I can see that isn't the case, but even had I read around the subject prior, I still would've made the same "mistake".
I should stress, I have no problem with our landlord, by the way. This is all handled via a letting agency, and you're missing an important piece of the story:
The reason we're so mad is we asked the letting agency, twice, how much money we would get back from our deposit. We explained we needed to know to plan our finances. They told us explicitly, twice, in writing (email) that we would get back 700€. Then we got a cheque for just under 400€ out of the blue.
Now, I'm sure you'll agree, regardless of rights, laws, positions, etc. - that *is* rude. 300€ is a decent chunk of our month's grocery shopping. Taken. Without warning.
Had the letting agency told us we'd get 390€ back, because the back-dated rubbish tax was to come out of the deposit, we may have been disgruntled that we hadn't been given the option to pay it monthly, but we would've shut up about it. But they didn't. They told us wrong, twice, then left us with a nasty hole in our budget.