On Monday, 16 April 2018 21:21:35 UTC+1, NY wrote:
Our bath/washbasin waste pipe emerges through the wall and then, via an
elbow, goes very slightly *uphill* for a run of a couple of feet, before
another elbow joint connects to a vertical pipe to ground level. The highest
point is probably an inch or so above the point where the pipe emerges
through the house wall, but as far as I can estimate, is below the level of
the bath plughole.

Is this likely to be responsible for water that is run out of the bath
ejecting the water from the washbasin U trap, or water from the washbasin
flowing into the bath and/or ejecting the bath's U trap wate, with copious
gurgling? And also causing very slow drainage of water from the bath
(several minutes to empty the bath).

I can see that even if it's not the cause of the gurgling and back flow into
the bath, it's not a good idea to have water standing in the pipe because it
will (and does!) freeze in cold weather: several times I've had to dismantle
the pipework and pour boiling water down the section that is normally
horizontal/uphill, to encourage the plug of ice to emerge.

A Bing search for the words of your Subject line, without surrounding quotes, would seen to offer sufficient enlightenment.

--
SL