On Mon, 19 Dec 2005 14:00:37 +0000, Douglas de Lacey
wrote:

Hi,

Try pouring a load of boiling water and some detergent _slowly_ down
the plughole then fill sink half full of hot with more detergent and
use full 4pt milk container to flush a load of water down followed by
the half sink full of water.

Wouldn't it be simpler to remove the trap and investigate?
Douglas de Lacey

OP has already done that:

On 16 Dec 2005 06:48:47 -0800, wrote:

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.

Sounds like the tenants could have been pouring fat down the sink, in
which case it may have solidified in the drain where the surrounding
concrete keeps the pipe cold.

Pouring lots of boiling water and detergent slowly down should heat
the pipe up and help soften grease, then a blast of water should flush
it away into the sewer.

cheers,
Pete.