If it goes "chickachickachickachicka..." and then starts, then the battery is good.
If it goes "..<click>...chickachicka....chi......cka...chicka...chicka.." then the battery is knacked... but on the face of it, the voltage 12.3V static is good.
When the car is running, measure the volts on the battery. That should be 14.5V or therabouts.
If the engine turns over quickly and the beast still doesn't want to know, then the most likely thing left is fuel (or rather, the lack of it).
If it starts okay when warm but is moody when it's cold then it could be the cold-start system isn't working properly. The ECU should inject some extra fuel when the engine is cold, so it may not be getting a signal (from a temp sensor?) that makes it do that, or there's a problem in the delivery.
If the car is just moody hot or cold, at random... you need to test fuel delivery (fuel volume) and the fuel rail pressure ... which unfortunately you need some special equipment for. Your local friendly Bosch Motronic Service Centre will be able to oblige.
But the cause could be a tired pump (or a split in the pickup line, in the pump), a blocked filter, or a dodgy fuel regulator. These would also affect how the car drives though.. and you didn't mention anything unusual there... but you might remember something (like you filled up with diesel a few weeks ago) etc.
Ralf S.