The ceramic bung you mention, Derek, may well be rodding access for an old type (not used today) "interceptor" chamber. If so, it will be above the trap level, so no smell should come out of it.
No smell should ever pass through a trap (U-bend).
The first, non-disruptive, things I would check...
Stand over the chamber and have someone flush the WC. Whether it flows normally or only sluggishly should tell you something. If slow, then it would indicate a serious break in the pipe run. The water may be saturating the area around it. Eventually, that would start to smell if where the soil pipe passes through the wall hasn't been sealed up properly.
Also ... put a drain testing bung into the entry to the chamber...
http://www.wickes.co.uk/Rothenberger-Drain-Test-Plug/p/118731?CAWELAID=120135120001173926&CAGPSPN=pla&utm_source=google&utm_medium=cpc&utm_term=&utm_campaign=Google%20Shopping%20-%20Heating%20&%20Plumbing&tmcampid=&tmad=c&tmplaceref=&tmcustom=mkwid|s_dc|pcrid|43364569612|kword||match||plid|
Flush the WC. The water level in the pan will rise to the top....... and stay there if there is no big leak. If the level drops immediately even after several flushes, then I would suspect a break. I'm having to make a few assumptions here. There maybe other branches and connections on the system, but it should tell you something.
It would be interesting to know what you find.