She didn't think it was the battery even though it's 10 years old and was reading 12.7V on a multimeter, because when she grabbed my truck and the jump cables, it still wouldn't start and just slow cranked...
But it seemed to have spark, and power to the fuel pump from her initial investigation, so we agreed to take a look together once I was home, thinking it might be ECU related or a relay.
After about five minutes initial poking around, I got my Jelly-senses started tingling, and I popped the multimeter that lives in her boot on the jump cables from under my passenger seat...
No continuity in the +ve jump cable...
Borrowed a NOCO jump pack from next door and the MX5 fired up first time, fitted a new battery and it's happy as ever.
I have been told to get better jump cables, and she's considering getting a little NOCO pack for her boot.