"We put a heavy stone on ours to stop any flies getting in there in the first place." OK, but that is not the full story, flies lay eggs on food in your house before you deposit it in the bin and they then develop into maggots in there. Here in Germany we have different bins and foodstuff goes into a 'Bio' bin, so you can't wrap stuff in plastic, though you can get biodegradable plastic ones. Try hard not to leave unwanted meat leftovers etc. lying about and make parcels wrapped in newspaper as quick as you can. Boiling water will kill them if you are unfotunate enough to have them in you bin.