I do not have a tall stack from my toilet but what I do have is a approx 1 meter vertical pipe with some sort of non-return seal which blocks any smell coming out into my sloping roof space. The system has worked well for the last 15 years but now and then a smell appears. To stop the smell, I flush the toilet a couple of times, and it clears. It seems the non-return part sticks and I do not know what type of system is fitted or what maintenance is required. Can anybody help in identifying the type of extraction system this is. - Frosty

This sounds like you have a AAV, air admittance vent,(see image) that's gone bad.
No problem, Easy repair for a homeowner. Simply remove and replace it. Good luck, Tom

This sounds like you have a AAV, air admittance vent,(see image) that's gone bad.
No problem,! Easy repair for a homeowner. Simply remove and replace it. Good luck, Tom

Tom
Thanks - Put 'air admittance vent' onto a search, came up with suppliers with reasonable prices. Screwfix at £16.99. Your answer was perfect - Thanks a million - frostypete

Thank you for the update. Is everything working now? Cheers, Tom