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.