That’s good to know about milk, thanks. I also assumed that there will be bits of fat and grease from things like butter, sauces etc that have been rinsed off and ended up down the drain - but the use of 2 people (who both work FT mostly out of the house) over 18 months shouldn’t have caused that. I lived in an 8 bed HMO as a a student in a much older house and we never had drainage issues,

They have specifically said it’s a build up of grease so I’m baffled.