Originally Posted by Kitchen Devil

It may be something very simple.

The fact the PATS light comes on and then goes off suggests that the key is being recognised, if it wasn't, the light would flash rapidly.

Check to see if you are getting power down to the solenoid on the starter motor. If so, good chance it's a starter motor fault so give it a bash and see what happens.

If there's no power, it may be a wiring or relay issue in which case it'll need investigation. Try powering up the solenoid manually with the ignition on and see if it starts. Make sure it's not in gear though!!

The rule is try to not overlook the basics, all too often I get called out to cars people think there is something major wrong with their car when it turns out it's actually a flat battery.