In message ,
Rednadnerb writes
Hmm.. block of flats with shared downpipes? Partial blockage at ground
level and then someone empties a bath.



--
Tim Lamb

Yes! that fits.
I'll get her to fill her own bath, empty it and see what happens.

er.. Needs to be waste water from a higher flat than hers.

Happened to a flat owned by my wife. Built mid '60's with kitchens and
bathrooms stacked vertically, with shared soil piping.

Fortunately the problem arose with the kitchen waste (sink/washer) slow
to drain and was fixed by rodding the pipe leading to the inspection pit
in the lawn.

--
Tim Lamb