Hi, thanks for the comments so far. Not sure if it has ever been ok, i
know it's been a problem for a while. it is in a rented house. the
problem is that they treat it like a bin and put all sorts down there.
I have dismantled the ubend etc and it seems clear all the way way till
the pipe goes outside.

Once outside, the problem is the pipe goes straight to the floor, i
think it is concreted in, therefore difficult to isolate what is
causing the problems.

I will go this weekend and try the milk carton and boiling water
suggestions.

Many thanks.