Just in case people come to this thread with similar issue, I managed to get the car going using a 12v leisure battery, and a cable leant against the terminal in the yellow circle (pic above) and negative to metal housing in engine bay.
Car came on and I was able to drive it.
Interestingly, I drove it for ~20 mins or so, expecting the 12v battery to have charged somewhat, but when I got back and turned it off, it was in the same state again.
So maybe I need that replacement batter after-all - but at least I know how to get it going.