Paste Flipped Pose Distortion

Hi, I have been trying to animate a simple model and rig that I made myself, which is probably the problem but I hope there is a simple solution. In creating a walk cycle I get to a stage where I want to copy and paste a flipped pose, but instead of the legs pasting in flipped position they cross at the centre (the wrong axis). I can copy a pose and paste it without any distortion.

Select a bone in edit mode, open bone tab, transform menu, set roll to 0 degrees.:o