Russ The diagonal cutout is actually at the top right hand corner when fitted,looking forward, the purpose of it is to clear the carpet attachment flange on the rear face of firewall which curves around above the top of the gearbox. regards
Now that it has been explained so well by you all, I can see that the drawing should be viewed in reverse ie through a mirror or through the the reverse side of the drawing.
Thanks for your help, as they say its simple when you know how.