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.
Thanks all for your help and advice.