Investigation tracked it to the overflow slot in the handbasin ????
Working on the premise the sink is rarely used, I disassembled the waste (bottle) trap, which was gooed up with a grey slime (which *was* the smell). Cleaned it all up, and proceeded to put the plug in, and put a tap on pouring a mixture of bleach and hot water downthe overflow.
All well for 6 days.
Now the smell is back - same place, but worse then ever.
The actual plumbing for the waste starts in the next room (the utility) where the washing machine discharges into a U-bended inlet. That runs into the cloakroom, where the outlet of the waste runs into it. From there it runs into the waste connector for the WC (which was made with a connector for waste.
So was my cleaning of the overflow in the basin not thorough enough, or could something else be combining to allow the smell into the room ?
Does it have any bearing that the basin is practically never used (just the 2 of us nowadays) ?