So just for context, it was starting, running fine and used daily prior to being parked up.

I noticed the alarm flashing LED wasn't blinking after a while, so it had completely drained the battery.
Took the battery off, gave it a good charge over night.

Not sure if there is a correct procedure to link up the key fob with the immobiliser, I locked and unlocked the car with the plipper, I just turned the ignition on and left it for a few seconds, repeated but noticed no fuel pump prime whir, obviously car just cranks with no fuel pressure.

It did splutter a bit the first crank but that could be just residual fuel left in the fuel rail / injectors but I didn't notice if the fuel.pump primed, I guess not?

Could it be somekind of immobiliser or anti theft issue? Would the engine crank in that case?

My car is fitted with the factory alarm.

Maybe the instrument cluster/ dry solder joint issue?
Are there any relays or fuses that would cause this issue in the engine bay fuse box?