My thoughts below, correct me someone if I'm obviously wrong

So... 9 out of 10 it runs. When very cold out it wont play ball.
Coil pack, plugs & injectors - probably OK (it runs on 4 when it decides to start).
Water Temp, Air Temp, MAP Sensor (built into ECU), TPS, O2/Lambda (if it has one in its exhaust, - Rob ??) - None of these should stop the engine running, faults should make the ECU substitute base values - ie LIMP mode.
Leaving the likely candidates....
CRANK SENSOR - now replaced. Very Probably OK.
CAM SENSOR - tells the ECU which is No1 cyl on compression stroke. This would probably fail to safe, and ECU would not start the engine. IF I was suspecting a sensor this is now it !
ECU - (as discussed at length with Rob) - ECU unpowered due to cold cranking amps being diverted/taken by the starter. Easy check with an ammeter.
WIRING - broken or intermittent make/break wire, connector fault, shield not properly terminated. Needs each individual ECU loom sensor & device wire buzzed through & continuity & sensor resistances checked off.
Anyway... all bloody annoying. I'm seeing Rob over New Year - will take a multimeter & if needed will borrow a mate & his PicoScope to check the sensor waveforms.