the cold weather can cause starting problems, ie; diesel not heating enough.
next time, go to start the van as normal [wait for glow plug light to go out] but WAIT a bit longer till you hear a click. Dont turn the engine over now, repeat the glow plug process another time or two, then try turning over.
to test glow plugs yourself, get a length of wire. put one end on the positive terminal of the battery, then touch the tip of the glow plug where the positive wire connects, if you get a little spark, the glow plug is fine, if not its knackered.
It only takes one faulty glow plug to increase the difficulty of starting