So, you're there, straining away, with palms pressed against the walls on either side of you and face red with effort.....here's what you do:-
get the childrens step (you know, the one they used to use for reaching the sink) or similar, and put your feet on it. Draw your knees together. Think 'Pixie look' IYKWIM
I don't know why it works - maybe it just gets apature, pipe and posture all lined up correctly or summat but anyway, poo comes straight out like a hare out of the trap.
Just sharing useful life-tip info.....