Thanks all for your very helpful replies. Gerson, we did pay Nebenkosten, but were always rather confused. We had been told when moving in that the rental price included all bills except for one (I can't remember if it was water, or whatnot). However, we regularly got bills from the gas company, etc. And every year, we were sent a bill for backpayment of what was explained to us as landscaping costs, heating, and who knows what else. The language barrier was a huge issue, as our German was only basic. We are expecting one of these backpayment bills later this year (the flat was vacated in August 2013). We do have a copy of the rental agreement. In the agreement I noticed that they said the whole flat should be repainted every two years, but looking online, it seems that they are not legally able to enforce this even if it's stated in a contract. Nevertheless, they did require painting to be done and we did it. A representative from the rental agency came over to check the flat the day before we moved out, and mentioned a couple of things that needed to be done, so we did them. This is why we are perplexed as to why there were charges for cleaning and painting! We were in the apartment for a little over two years. The contract had to be cancelled early, but another tenant was found, so all is well in that regard.

My partner and I lived in Germany for a couple of years before moving back to the UK. We followed all of the rental obligations carefully, and were shocked when the rental agency hit us with ridiculous and exorbitant fees for cleaning costs, etc. even though the flat had been meticulously cleaned before we left. Unfortunately I was overseas for awhile, so my partner dealt with all of the fallout. He felt hopeless to dispute it, so he paid what they asked (I wouldn't have, without a fight!!) and then they also took the whole rental deposit, meaning that we've paid over €2000 despite all of our careful efforts to follow the rules and contractual obligations. We're worried that they might send another bill later this year, for last year's utilities, and we just can't afford it. Yes, in some way it does feel irresponsible not to pay a legitimate bill, but we're so angry and frustrated about the other unjustified expenses that we want to avoid losing any more money. So my question is, if the rental agency sends us another bill of less than 500 euros, and we don't pay it, will there be any serious consequences to us in the UK? Surely they will forward the bill to a collection agency, but as long as there wouldn't be any impact on his/our credit rating, or UK court involvement, then I suppose it doesn't matter too much. I should also mention that we're not going to be moving back to Germany in future.