Just realised you are talking about a Discovery and my problem was on a Defender so there will probably be a few hundred more lights involved and the temp guage symptom won't be the same as I think that reads in the opposite direction on a Discovery.

What lights and behaviour do you get when it isn't starting? I had a similar problem with a (new to me) TD5 over the summer which developed an intermittent starting problem. I'm new to all these electronics in a landy so I was on a bit of a learning curve trying to work out the cause. My...