So I solved the problem. As suggested here, it was indeed the mirror clips holding the primary in place. Initially I thought it couldn't be as the flares were asymmetric and didn't seem to match the clip pattern. However, the problem was disguised because my secondary wasn't even close to sitting centered under the focuser tube. It was only after I got a sight tube that I realized this - so a laser collimeter alone is not good enough!
I could collimate the scope with the laser and get decent images, but because the secondary wasn't correctly centered some of the clips were not part of the image reaching the camera, hence the asymmetric flaring.
After centering it was immediately obvious that the clips were the source of the problem. I made a crude circular mask out of cardboard to confirm and placed it over the primary such that it covered the clips and the results speak for themselves.
This image is pre-secondary centering showing the original problem (focus is poor here, this is while troubleshooting):
This one is after centering the secondary and fitting my cardboard 'donut' mask.
Thanks for the suggestions, glad to have gotten this sorted!