Found it.
It was the distributor rotor button. Of course it was the last place I looked. I only pulled it off because I noticed an abnormal notch inside the distributor cap which looked like it was caused by the tip of the rotor button.
The motor came out of a write off, it looks like the distributor was pushed up against the firewall and the rotor hit the cap and stripped the plastic key out of the inside. So it was in an incorrect position on the distributor shaft. Even though it timed correctly and idled ok, i'd say it was out of sync with the hall sensors and hence started playing up when the ignition tries to advance.
Replaced it and all good!