wrote in message
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The only thing that immediately occurs is to use a larger bore of pipe for
as much of the run as possible.
i.e. have large bore pipe from the sink and/or bath right up to the 40mm
socket in the trunking.
I think that this would ease the flow of the water by reducing the
resistance along most of the pipe; having said that I don't think modern
baths drain well at all.
ISTR that baths in old houses (and old hotels) used to drain much faster.
I have guessed that this is due to larger bore lead waste pipes in old
installations.

Other option is to raise the bath with a step next to it to make getting in
and out easy (opposite of a sunken bath).
This would give you a better head of water/drop over the length of the pipe
run.
You could also raise the basin by putting it on a raised base - but this
might look wierd :-)
Think of it as a part raised floor in the bathroom.

Combine the two - have a large bore pipe under the raised bath.

Our main bath drains slowly, and the waste just runs the length of the bath
(taps away from the outside wall) so I think you will be lucky to get
reasonably fast draining with anything but the shortest run.

Having a rambling day today :-)

Dave R