Part 1 - My Defender has died with an electrical issue!!

Having not used the Defender for a couple of weeks, I decided to start it and warm it up. Things were going well until it decided to run on what sounded like 2 cylinders rather than all 5! It now no longer starts, no glow plug light and the temperature gauge reads really hot, buts its minus 3 outside! In part 1 I'm investigating a possible cause after seeing similar issues people have posted on forums and how they fixed the problem