Syphonage of kitchen sink trap?

[Slugbabydotcom]

A hepVo valve wont be any good.
There is a water seal in your trap to stop smells getting back up and theres always a drop of water left in the pump on your washing machine which again acts as a trap and stops smells getting through there.
Is the wall patched up properly around the waste pipe?
Are there any other vents or holes where the smell may be getting in.
If things are as you say then its likely that a soil pipe belongs in the place where you think you have a gully.

 

[trotter]

So you don't think the smell is getting up through the sink at all? Obviously I know there is a water trap in the sink but I was assuming the trap was being cleared somehow...but I may have been barking up the wrong tree all along then

 

[Slugbabydotcom]

It is possible that a trap can self syphon if say there was something like a mop string going from near the bottom of the trap and over the outlet part.
Its worth taking the trap apart just to rule it out.
To see the same effect leave a glass of water on your drainer with a wet string type cloth that hangs over the side. Its an amazing phenomena courtesy of the effect of capillarity