I just had the same problem. To remove the lock cylinder you are supposed to turn the key to position 1, but if the lock is siezed up that's no help. So what I did was as follows:
1) Order a new lock cylinder to suit your key from a Vauxhall dealer. It will cost about £51. Take all relevant paperwork, ie Log Book, Security Card, if you have it, and any other proof of ownership with you. I ordered mine on Friday, and got the new one on Monday.
2) Remove airbag, steering wheel and covers. I found out how to do this in the Omega Haynes book.
3) Use some judicious force, ie hammer and medium screwdriver to break off the key end of the cylinder, and remove all the innards. TAKE CARE NOT TO DAMAGE THE BARREL WHICH IS PART OF THE UPPER STEERING CLAMP
4) Then you will be able to turn the lock cylinder to position 1. Insert a small allen kee or other tool to depress the retaining spring at the top of the cylinder through the hole provided in the barrel. Then you will be able to withdraw the lock cylinder.
5) When fitting the new cylinder be careful to ensure that the hole in the centre of the electrical switching contact block is perfectly aligned with the rotating spigot sticking out of the end of the new lock cylinder. You must also ensure that the steering column anti-theft lock is not engaged, by depressing the latch thingy at bottom end of the barrel.
The Haynes book describes most of the above, but it also suggested removing the electrical contact block, which I would advise against. When I tried to do it, the whole electrical contact assembly fell apart, and I had a job to reassemble it correctly.