Hi folks
I have the same problem. Smell coming through one sink in particular. However I noticed it was worse on windy days and sometimes worse when the wind was in a particular direction. I went for the high tech solution. I  put two plastic plant pots over the breather tube, you can twist them round to regulate how much room there is for the wind to blow through. It's loose fitting and allows the breather to work. Up to yet, touch wood it's working.  Worth a try.

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