I'm a PhD student in Computer Science in the UK. I had co-authored a paper with a Lecturer from another university (not any of my supervisors, just someone's work I'm building upon) in my first year. I have kept him up-to-date with my work and he's very familiar with what I'm doing. Would I be allowed to have him as an examiner for my thesis?
Edit: Just to clarify, I'm referring to an external examiner.