Smell from bathroom drainage

[paul_handyman]

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 or the shower. I have not noticed it when the bath 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.

Any ideas what could be the cause of this smell?

[TheDoctor4]

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