So it turned out the intermittent drain whiff SWMBO (and I !!!) have noticed these past few days is because the outside drain from the kitchen (waste pipe goes through wall into drain) was blocked.
Past tense, as I have managed to clear whatever it was with a good prodding with a piece of dowelling ... prod, prod, prod, sudden "flump" and the water just disappeared.
Have run a lot of water through, and will put some caustic soda down there last thing tonight to give it a chance to dissolve any gunge.
(Have 2x containers of more NaOH on order from Amazon).
Because the drain overflowed, there's some whiffy water surrounding it. This is under cover so it's less likely to be washed away (by the endless rain we seem to be having).
2 Qs.
1 - Now the source of the problem has been addressed - for now - will that water pretty quickly soak through the paving and leave no whiff ?
2- Having splurged whatever the blockage was, and put soda down, is there any more to do, or should it stay clear now.
I suspect we've been a little too cavalier with FOGs :( More care in future .....