Another starting issue post

So heres a strange starting issue for you all, the car has been sat for a number of years and I've decided to get it back on the road.
ive fitted new leads, plugs, dizzy cap and arm, catalytic converter, battery, fuel pump, fuel filter, oil, coolant, oil filter, air filter and oil pressure sensor.
The car cranks fine but will not fire whilst cranking it just cranks and cranks and cranks. I've checked spark and thats fine. Also the fuel pump can be heard priming just fine.
However, if i just flick the key on for almost a second and then let go it fires up. But sometimes can take a few tries

What could this be ? Any ideas ? I'm pulling my hair out over this Thanks
 

Immobiliser?