Good to know that your found the cause. And a simple one at that. Whew.
I had a similar situation, no go on the YANMAR, for no apparent reason.
It turned out to be the isolating solenoid on the 12V -ve side was sticking. Easily overcome, as the solenoid has a manual override - a rubber bell housing covers one end of the solenoid, allowing you to manually activate the solenoid with the press of a finger. This does however require one person in the engine room, and one at the helm - that is, unless you have one of those snazzy by-pass do-dads as elsewhere recommended. Once the solenoid was activated manually a few times, it would then work by itself when the start circuit was activated - and a judicious squirt of CorrosionX has seen no further problems in this area (thus far).
Curaçao, about to set course
tomorrow for Bonaire