My wife had long complained that the sink drained smelled strongly, particularly when hot water was poured down.
Endless use of bleach had no effect. Initially I though it might be due to the long runs of exposed pipe above the combined trap.
But replacing this arrangement with two separate traps, directly below the waste fittings, made no improvement.
However, taking the sink waste apart revealed the problem.
It was clear that the 20mm vertical gap left between the two halves of the reducing waste adapter was harbouring dirt that was out of reach of bleach or any mechanical cleaning method.
I had some 50mm bore flexible waste pipe in my (extensive) collection of odds an ends, and thought this might be cut to make a narrow lining for the cavity; a lining that would flex vertically as the two parts were tightened together, but also be stable laterally. Note that it is reinforced with a plastic "wire" spiral, not a metal wire that might corrode.
It seems to work:-
I'm now waiting for the verdict of the person with the more acute sense of smell
in the house...............
But it does make me wonder how may other households have the same problem, and
whether there is not a opportunity for a product without this weakness?