Diagnostic plug?

That car and year@not sure,
first place I look on these is under where the radio/heater would sit in the centre consle,kinda align your head along the h/brake when off ,lokking to the front,with a torch,and you ought to see to socket.
If it aint there then again around the h/braken there is a plastic insert round about where your thumb would be if extended as far away from the lever and straight down.
I keep a little hook/screwdriver to hook under the plastic and pull up.

Not sure on this vehicle, But the Mk4 astra is at the base of the Handbrake under a plastic insert that you lever out with a small pick screwdriver. The Omega is under the steering couling. looks like a glovebox. But has fuses and relays in it with the diag plug. They do like to hide them. Hope some help. If not Google it. Or call a local garage. Doubt Vauxhalls will tell you maybe try Halfords. Or a AA or FAC man on your journeys. Cheapest way

under the dash under the steering wheel dash cover

not sure on this 1 but if its any help eu rules state that all cars entering the eu sales market must have the diagnostic plug with in 18" of the steering wheel