I thought it was a blockage and got a plumber out but he said there was nothing blocked and that i needed to replace the durgo valve, as when he took it off it all drained fine.
I have replaced with a new one (had to cut the old one off as it was stuck on and have now bought a push in one) however....the same issue. The valve works fine (ie when it turn it upside down it moves). But if the valve is in the pipe the toilet wont flush properly and air still comes up through other outlets (toilet, bath) not the valve. When I take the valve off the pipe everything drains fine (but of course I can't leave it off or it'll stink.
I have no idea what to do or why this is happening?! please help