I'd say it's the air admittance valve that's letting the smell out. You said earlier on that you dont use the bathroom that often, how long will it go between uses? I'm asking because if it goes long enough then the water in the trap will eventually evaporate and will let smells pass through it.
Get youself this PIB and replace that air valve that should sort it..
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Would all bathroom plumbing be 40mm as standard? I just noticed that there's also a 32mm one on there, and want to make sure I'm getting the right thing.
1-1/2" 40mm on bath waste mate..I see a nut on your waste near the AIV ready for it..
I loved this thread, better than sos diy,I learnt a lot and even had my bath panel off.
Looking forward to part 2 check your roof.