Hi McRoth,

You got some great suggestions above and there is little I can add.
Hovever, for the sake of irony, I recently went through the same situation.(same car too, 82'240d manual)
And like you, my car started great last winter in well below freezing temps.
This winter different story, glow plugs were not much more than a year old, valves recently done. Brand new powerful grp49 Interstate battery etc. And car would start right up at 50+deg. and below 40deg. I had no wheels....

I say, if you still have trouble after you've checked valves etc.
Put aside the fact that your Glow Plugs are new. Take them out individually and put 12Volts to each and watch them get bright orange.

It was intesting what Jim stated above(and more than likely true)

"From the description you have given, there seems to be a temperature threshold that affects the car's willingness to start."

This was exactly my situation, as all the components needed to make great compression were working fine, but it still would not lite-off. Why? Because despite my earlier quick tests to check glow plugs in the end I found 2 of 4 totally non-funtional. Hence the air and block temperature is critical for combustion.
Bottom line... MAKE SURE all your GP's work.
Sorry for ranting,
good luck
dave