Chris Green wrote
| The bath is in a flat and is positioned about two or three meters
| from the main service duct where the sewage downpipe is. The
| waste pipe from the bath runs essentially horizontally from the
| trap under the bath across to the downpipe. Not surprisingly,
| especially towards the end, it drains very slowly. It's impossible
| to get at the connection of the waste into the downpipe as it's
| inside a brick built service duct, all you can get at is a 40mm
| 'socket' where it pokes through a hole in the brickwork.
| Can anyone suggest any clever ways to improve this?

It's not clear if the bath outlet runs within the floor void or if it *has*
to, but if you could elevate the bath by a couple of inches (say by using a
couple of floor joist -like bits of wood on their sides as planks under the
bath feet) that might give you enough more fall on the pipe.

Even if the pipe has to stay within the floor void, elevating the bath a bit
should give an increase in flow because of the increased 'head', thus
encouraging most of the water out a bit quicker and reducing the amount that
sediments/stagnates in the flat bit of pipe.

Owain