Like a wall.The whole thing is only 2 or 3 years old.
The mouse theory was discussed before, but then, like I say, it can be ok for weeks, that's what throws me.
It is ALWAYS worst in a morning, and if we try heating on in an evening, I would say it doesn't stink.
Think there may be an evaporation issue, maybe???
Frustrating me now I have all your suggestions, as I'm away at work at the moment, and can't get stuck in to have a look for myself.
Thanks for all your theories tho, all very welcome.
I could give you a few more to check is there a vent on the 4" pipe outside,