I got no replies last time so I will ask again -
I have found quite a few posts about smells from bathroom drainage but all appear to have the smell in the bathroom.
My problem is different in that the smell appears outside in the drain.
I know it is not the drain itself - I have checked that by running water into it from a hosepipe. We also do not get the smell when any of the kitchen waste is run into it.
The smell (like a really foul drain) only happens when the washbasin appears to be used . I have not noticed it when the bath or the shower empties.
The plumbing has been in place about 15 years but this smell has only started happening recently.
I thought at first that it was the outside drain but, as I said, this has now been ruled out.
There is no smell appearing in the bathroom itself.
I have checked the U-bend which is clear.
Any ideas what could be the cause of this smell?
