Battery is a good start but;
I would go for the Camshaft sensor.
Barometer readings would not prevent it from starting.
Same with Turbo boost pressure I would think.
Why does the engine start with “Start aid spray”?
IF the ECU does not get a signal from the Camshaft sensor,
It will NOT inject any fuel.
No Fuel, No go.
Easy Start Spray replaces the missing fuel and the engine fires.
Magnetic hall sensors produce a better defined signal if the magnetic field is manipulated violently.
Strongest signal is around 1000-2000 rpm.
If the Camshaft sensor is on his way out, the signal gets weaker and the ECU can’t see it.
If you where to tow-start the engine, at around 1500 rpm your engine would most likely start.
On your 16 Valve M-Jet, it should be easy to fit a new one.
Cost is around £40 for the Camshaft sensor over the internet.