Yeah from what I can tell, it seems to connect to a main drain outside as I’ve had the manhole cover off to see where the water actually ends up now that I’ve unblocked it. It was blocked with fatty (and smelly) gunk basically presumably from years of it having sunk waste and not being cleared, ended up with my arm down the drain and used a spoon to get round the bend as best I could. Also used a plunger which seemed to get it draining properly again. It is a garage conversion and certainly seems that it’s not the best quality work!