Morning Paul
Diesels fire on compression not ignition so require good fuel pressure, good compression and good cranking speed with those the engine will run. In cold weather the Glow plugs raise cylinder temperatures to aid fuel atomization and help starting.
So if I were checking I would begin with the battery, is it in good condition and fully charged, only charge from the charge points under the bonnet or remove the battery from the car. Check the starter is in good condition Magnetti starters did cause issues at one point with E53's. Is fuel pressure correct (when was the fuel filter last changed?) I would still check the Glow plugs and Glow Plug controller and loom for chaffing or cracks.
Dave