Almost definitely a loose connection in the battery - I did test it for continuity and that was intermittent too.
One possible contributing reason for failure was losing the metal battery strap on a run, so it was bouncing around under the seat for quite a few miles.
I decided to use an old black leather belt as a strap as it allows a bit of movement and you don't risk cracking the battery casing. It fits really well, going through the front and rear slots of the battery tray. I've also bought a really thick rubber mat for it to sit on so hopefully no repeat.