Started yesterday morning & ran fine, switched it off after 15 mins & won’t fire up now
If you have a spark, then both the immobiliser and crank sensor are OK.
So there's a very good chance You've flooded it.
Ensure that the battery is fully charged, remove the fuel pump fuse, and depressurise the fuel rail using the test port. Now with the throttle fully open, crank the engine on the starter for about 30 seconds, or until it starts to slow down. Do this at least twice. Then refit the fuse and turn the ignition on and off a couple of times to reprime the system. Then try to start it normally.
If this works, drive it for several kilometres before turning it off again.
My experience of these cars is that they take forever to warm up if just left idling - after 15 minutes idling, the temp needle has barely moved, and that's with a good thermostat.