House (and drains mainly) 53 years old, but this smell is recent problem.
All possible causes have been investigated thoroughly & eliminated as possible causes. The "cloakroom" is actually a full (downstairs) bathroom. Basin & bath have proper traps and, in any case, drain into a gully in the adjacent garage. This gully has a trap and is not smelly. The ends of the wastes from the bathroom are well clear of the water in the gully trap.
I can therefore suspect only the WC as the source of the occasional, and randomly occurring "nasty smell" in the bathroom, but there are no leaks from the pan. This has an S trap and connects to a drain pipe running underground, via an inspection chamber, into the main sewer. The drain turns in the proper way below the WC connection via a slow bend.
A clue might be that we occasionally find the water in the WC trap ( a large trap - this is a Twyfords "Classic" pan fitted in 2007) low, but not usually below the "partition". So this occurrence is not necessarily connected with the smell, which continues to seem to have no obvious cause. In fact, VERY occasionally, during prolonged very heavy rain, our main sewer may back up. Back pressure, once the backed-up water covers the inlet in the inspection chamber leading back to the bathroom, compresses the air in the drain from that room, lifting the water in the trap. Curiously, even though this sometimes blows through the raised water (which subsequently, of course, drops low in the trap), such occurrences do not necessarily coincide with SMELL episodes!
I wonder if the basic problem is that the WC is connected direct to the drains. There is no soil pipe open to atmospheric pressure at its top end to relieve back air pressure in the drain (though why should there be back pressure in the drain which is vented to atmosphere by the main soil pipe serving the upstairs bathroom?).
Would the rather daunting task of fitting a soil pipe to the WC drain from the downstairs bathroom (it would vent to the atmosphere as soon as it has passed through the flat garage roof) avoid this SMELL EPISODES problem? Or is it just an occasional nuisance which we have to learn to live with (though, having lived in this house since 1979, it's very strange that we have noticed this only in recent years)?
