It seems 300tdi's fall into two categories, the easy cold starting type and the difficult cold starting type. I have one of each, both NON EDC.
Last month, it averaged about -5°C for about 3 weeks, dropping to -15 at night, I could start the 90 no bother every time, I did also deck the clutch as the drag from the cold gear oil is horrendous. The Disco is just a bit harder to start, always seems a bit sluggish.
90: Key in, 7 seconds on glow plug button (Bosch plugs), throttle and clutch to the floor, turn key, engine catches almost immediately (immediately when it's not as cold).
Disco: Key in, turn to glows twice (not sure what make are in - but certainly tired), throttle and clutch to the floor, a bit of sluggish engine turning, then she catches.
The Disco could do with having the earths all removed, cleaned up, and refitted.
So first on the list, glow plugs, ALWAYS buy BERU or Bosch; Bosch are OE, not only that they are probably the best you can get. The last Bosch set I bought had a specification of 5.4 seconds to full heat output (based on 11V and ambient being 0°C) even being well below 0°C will not stop these getting up to temp within probably about 6 seconds. So if you have old or poorer quality GP's and you have a glow timer relay you need to consider that one glow period might not be enough for the plug to actually get incandescent hot. I have removed old glow plugs and bench tested them and found them to get up to temperature OK but after about 15seconds, so turn the ignition off and give the glows 2 or even 3 goes, as soon as the light goes out turn the key.
Deck the clutch to reduce load on the starter motor, the cold oil in the engine is bad enough.
Deck the throttle (note: this technique will NOT work on a LUCAS injection pumps, only the bosch pump as fitted to 200 and 300tdi LR engines) this puts the Bosch pump into excess fuel and will aid starting.
Don't let go of the key till it's caught.
If the starter does little or sod all, then the aim is to get some volts and current to your starter. Cabling or battery, you work it out but if you have all of these things, the 300tdi should start.
If it has all these things and it is still a pig to start, then there is something not right.