Just managed to get a better look at the waste stack. I think the problem is that the 1m run from the back of the toilet to the stack starts off with modern plastic wide pipe and then connects to what I assume is a narrower 1930s cast iron junction with the main waste stack itself.
Bolted.
Just managed to get a better look at the waste stack. I think the problem is that the 1m run from the back of the toilet to the stack starts off with modern plastic wide pipe and then connects to what I assume is a narrower 1930s cast iron junction with the main waste stack itself.
Basically there's a bottleneck.