After spending many hours replacing glowplugs, fiddling with relays and other bits and pieces, truck still would only start intermitently, very annoying. It seems the culprit for why my glowplugs seem to work when they fancy it is ... the drivers door switch sticking! How annoying! Its a bit like a 'secret immobiliser' now as my truck only starts when the door switch is fiddled with!

Ah yes the mk3 golf used to do that - the glow plugs heated when the door was opened so they were already ready for when you turned the ignition on.