Our downstairs WC sometimes gives off a strong 'sewer gas' smell ....
> Alternatively I am wondering if the 90 degree plastic pipe might be not
> fully sealed - but absolutely no water leaks.
Ours used to do the same - I thought it was the toilet area smelling, but I finally realised what it was one day when I'd sprayed everywhere with cleaner, and it started bubbling at the top of the rubber joint where air was getting out. I took the bog out and fitted a new seal (around £5 and a hours work), and it has been fine since. I think they must perish/get hard with age, as they get varius cleaning fluids and bleach sprayed around them every so often which cannot do them any good. Alan.