I may have found the culprit. The fuse-able link I'd installed showed signs of arcing on the back of the ring terminal which indicates it was not making a good enough contact for the current surge when starting the engine but may have been ok for low power stuff and charging the battery. I've removed the link and bolted the two ends of the cable together. The engine has had a few successful starts with no issues seen so I'm hoping that's the problem solved. Only time and testing will tell though.
