Hi dextrous, it seems so bloody obvious now you point it out, but I think that is exactly what the problem was. Doh...
We only moved in a few weeks ago and that's how it was left.
The smell has almost instantly disappeared after following your suggestion. I can se where the tube dips into the water and where the 'O' ring creates the seal in the trap. Albeit, most of the chrome has now pealed off the sides so it's not a perfect seal, most poss due to all the bleach we have poured down there since moving in. Hence, I may still consider changing it, as it's such a cheap repair. I'll confirm in a day or so, but I think you may have nailed it.
Thanks to others who also suggested the 'O' ring may be at the wrong end.
cmother1 - This is a typical George Wimpey new build. This is professional plumbing right before you. That waste is brown plastic 40mm and I reckon it does go straight to the SS. However, I'm not digging up the chipboard floor to find out.
If you shop around, you may well be able to buy an identical trap, which you can use the grill and chrome surround for, and chuck the rest of it away. Bit of a waste (no pun intended), but a lot easier than replacing a perfectly good trap that's already there.
Its fitted on backwards The white bit works as a trap seal and is removable for cleaning. The way it is fitted you have no trap seal so the smell just passes by. Btw that trap is only a 19mm seal on, not too clever.
The bit at the bottom of the pic that you can grab hold of should be at the top and the chrome grating does not attach to it.
Pull the white bit off the chrome bit, lower the white bit into the trap O ring to the top. Put the chrome bit back on the grating.
Probably fitted by a halfwit! If it is a new house it would have been subject to a drain test
Should`ve stayed with burgers Magical Mystery Test on the drain too. If they had used a Hep V O waste valve in line and removed that offending piece from the trap..there would never have been any backflow of smell. And it would have passed a proper air drain test. I don`t see how a 19mm water seal could pass the test, But what do I know. Last time I was on new build it was a small development when I was apprenticed in 1973