Master preheating lamp issue

That sounds familiar! I had a similar issue with my Master. The preheating unit was the culprit. Have you checked the cable and plug connections to the actual unit itself? That's where my problem was. All that engine vibration can loosen things up over time. The glow plug light staying on is a dead giveaway!

I had a quick look, but maybe I wasn't thorough enough. Whereabouts is the preheating unit located exactly? And what should I be looking for, like, is it obvious if a connection is bad?

Okay, so on mine (it's the same model), the preheating unit is near the battery under the bonnet. Give all the connections a wiggle; you might find one that's loose or corroded. Honestly, though, if you're not comfortable, it might be worth taking it to a workshop. They'll have the proper diagnostic tools to pinpoint the issue. Could save you a lot of hassle!

I took it to a local garage and you were spot on, a dodgy connection to the preheating unit. They cleaned everything up and replaced a bit of wiring. Cost me 95€ all in. Runs like a dream now!