Ive recently encountered this problem. Got the nuts off on the OSF but then couldnt get the NSF off, as the key kept slipping. I think that one of the prongs has rounded a little, so it cannot get a grip.
Took the car for a service at a friends garage (as its only just been purchased), he said that they managed to get the OSF and NSR off but the other 2 wouldnt budge, though presumably they havnt tried to the brute force route. Said it would be best to contact Vauxhall, which i did.
They said, as mentioned in this thread, that if you have the box they have a 3 digit code number which corresponds to any of 100+ different designs. I headed to the workshop though and someone there claimed that the key i had is not a geniune vauxhall key and the only way therefore would be to either find the original key (which isnt with the car!!) or they will get it off for the cost of an hours labour, about £85. Not likely.
So that leaves a few options. I saw the links further up for halfords and screwfix, are these any good? Does anyone have any experience with the nuts on an omega?
What type of metal are they made of? I know a few welders, can i get someone to weld the end of a normal wheel nut on the end and get it off that way?
Does drilling into the nut have any sort of positive effect?
In response to MickAP, i couldnt be bothered to go back outside into the rain to take a picture but it looks broadly like this;

P.S. Is the code for the keys not stored on a database somewhere, with the VIN and such?