The NOCO Li-Ion starter has arrived… and wouldn’t start the Mantis. It did start the Mini One however, so the NOCO unit is OK.
The CTEK battery charger/tester also arrived. I removed the Optima Red Top battery from the Mantis, & it was at 4.2V across the terminals. Left it on charge from the CTEK & it got to 11.6V but the CTEK & battery were both getting pretty hot so I disconnected it. A day later, not having connected it back to the car the voltage is back down to 5.4V. 😭
At this point I have decided that the Red Top can’t be reconditioned or brought back to life & I don’t fancy cooking my new CTEK charger either. So now I need to find an alternative battery which will fit in the tray & will last a little longer. And, of course, figure out what is draining the battery.