Thanks guys. Seems that it was a failed battery after all. Rechecked the voltage with ignition key to 'on' and multimeter reading dropped to 0.6V from 12.5V when in 'off' position. Really surprised that it died so suddenly as it had given no sign of failing and thankfully it got me home before it croaked.

Connected up to car battery from jump cables and everything lit up and she fired first time. Just wish I'd done that first but the battery tender test/charge also seemed to support an ok battery.

Really pleased that it's an easy fix.

Any recommendations for a good replacement and with negative terminal on the right when viewed from front - Yuasa AGM or a Li-ion? Dead one is a MF CTZ7S.

Many thanks again for the advice!