I have a bathroom with a bath, two sinks and a WC. When I run either sink for any length of time water starts coming up through the plug hole in the bath! Happily this does not happen when the WC is flushed - though the WC is immediately next to the soil downpipe and connects directly into it. Eventually the water which enters the bath does drain away.
My conclusion from this is that there is a partial blockage in the underfloor pipe which leads to the soil downpipe after the place where the closest (to the downpipe) sink joins that pipe. Thus water runs out of the sinks, hits the block and backs up into the bath (which is the path of least resistance as it is lower than the sinks). Having looked at the posts to this group, it seems that I should put a quantity of diluted caustic soda down the sink in question with a view to it disolving the blockage. Does that sound correct?
Many thanks for any help.