Thanks for the posts.

After reading your comments I had another look at the layout:
The top of the soil stack is about 3m above the wc connection however
about 18 months ago I fitted an extractor fan. Due to coombed ceilings
I chose a 12V model about 1.6m from floor level above the wc. However
this is only approx 1.5m from the head of the stack.
The date of installation roughly corresponds with the appearance of the
pong.
Our shower is over bath so, along with the presence of a teenage
daughter, there is little chance of traps being dry.
I will disable the fan, seal with cling film for a few weeks and see if
the smell returns.
Thanks again,
Ian