ianw wrote:
periodically we experience a foul smell in the bathroom (not just after
a recent visit!)
I suspect the soil connection to the wc pan
the current connection appears to be clay (?) wrapped in some kind of
bandage which dissappears horizontally into the wall.
The exterior stack is cast iron.
the clay pipe is obviously very old and does not even have a circular
cross section any more (but there are no signs of a leak this side of
the wall)
Would a new plastic pipe with rubber flange simply push-fit into the
cleaned-up cast iron?
Could the smell be caused by a blockage in the top of the soil stack
(birds nest etc)?

The house is a two storey stone building of 100+ years

What is the level of the vent?

On an old property too, there may be an open junction just outside the
loo where other plumbagoes.

Perhaps a picture of the outside with the offensive window exed would
help. If it is odour coming from the top of the stack, the weather is
likely to be similar in all cases where the niff comes in.

If it was a leaking joint then the stink would be deadly and permanent
and you'd soon notice where it was.