I wonder whether these tenants don't have English as a first language and therefore have no idea about recycling and rubbish being separate?
If you get on fairly well with these young men, can you talk to them and try to explain - even in pictures or drawings if their English is poor?
If you are sure their language skills are good, then it's up to the landlord to make sure rules are kept on the property, although it still might be worth having a kindly word to one of the tenants and pointing out the health hazards.
I would hesitate to grass to the Council, or make any nasty, hasty moves. As you say, there are worse neighbours.
When I lived in Bradford many years ago, we had a house opposite in which, it seemed, 20+ people lived in 3 bedrooms.
As it turned out, each room had 2 or 3 sets of bunk beds and when one lot of young chaps came off shift at the local mill, they got into the bunks, their fellows had just left.
They were a fairly quiet and polite lot and the house was well kept, although the sheets on the line outside looked a bit grey!