Thanks for the responses.
Onetap - the soil pipe from the toilet runs internally in a boxed-in section and goes down through the kitchen and out of the wall below kitchen floor level.
The smell seems to be worst at a vent on this boxed-in section at the back of a kitchen unit, although...
After about a year of bad smells in the kitchen, we got a drain specialist in who charged us seventy quid for swilling some water round our drain but didn't offer any long term solution to the problem which is...
On flushing the toilet (upstairs), the sewage comes down very quickly...