23 should be warm enough to start.
Get some heat in the engine bay - maybe a few 100 watt light bulbs (if you can still find them...) One by the oil filter, one by the IP and one by the turbo [more heat the better]
Get some heat on the oil pan - a tray of charcoal should be a good source to heat and thin the oil [more heat the better]
Maybe do a filter change just to be safe. [eliminate clogged filter or ice as the problem]
Get a jug of good diesel and disconnect your supply and feed lines and put them directly in the jug like you're doing a diesel purge. If the fuel is at all thick, add a bit of RUG to thin it out. Crack your injector lines to make sure you've got fuel out of the IP [eliminate bad fuel and tank strainer and ice as the problem]
Double check your glow plugs [even new one can go bad, and even one failed one will be a problem]
Make sure your glow plug relay is working [check the fuse on the GP relay]
Hair dryer in the intake during cranking [the more heat the better]
I guess if all else fails you could use the Mongolian Block Heater trick...
