Engine will not fire

No fuel
Check there is fuel in the tank – don’t believe the gauge!
Loosen the bleed screw on the fuel filter and pump the priming bulb. Fuel should come out. If not then suspect a faulty priming bulb/a blocked fuel pipe or a blocked fuel pickup in the tank.
Loosen the injector fuel supply pipe unions and have an assistant turn over the engine. – A good strong spurt of fuel should come regularly from each union (do not get in the way of any fuel spray – it is dangerous). If not then suspect a faulty fuel shut off solenoid on the pump, a broken fuel pump belt (unusual), or a fuel pump failure.

Poor/non starting only when cold
Check glow plug relay is giving power to glow plugs (check for 10-12v at glow plugs when the igntion is turned on with a cold engine). If not then suspect a burned out relay (quite common)
Check glow plugs – Disconnect battery, and disconnect wires from each glow plug. Measure resistance from earth to electrical connection on plug. It should be an ohm or two. If it is zero then that plug is faulty. If it is infinity the plug is also faulty. An L series will start fine on 3 glow plugs, the forth does not need changing as it is a pain when hidden behind the fuel pump.
If the poor starting occurs after distrubing the cam or fuel pump belts then suspect incorrect fuel pump timing as the cause of the poor starting.

Engine turning over too slowly
Charge the battery and investigate why its charge was low in the first place – Battery voltage with the engine running should be 13.5-14.5 volts.
Also check battery and earth connections.