Andrewberry it used to work fine on my 13 plate Zafira. Vulcan, press both brake and accelerator flat to the floor then put the key in the ignition and turn to the on position when the dash lights up but do not start the engine. If there are any stored codes they will be displayed where the mileage counter is. Or they may be displayed by a series of flashes. There are vids on youtube showing how to read them
I have this problem too. However if you have your foot on the clutch and slightly press the gas pedal the car will start with no issues, i thought it may be due to the starter motor but apparently Vauxhall said mine was done. Then I thought I might have that dreaded oil leak but again no, I'm unsure what it is at the minute but pressedign those pedals seem to do the trick.