Hi, new to these forum things and hope I'm not too late, but did promise to let you know of cost of repair. After trawling around, the local Vauxhall dealer seemed to be the best bet, and also very knowledgeable about the problem. "All Omegas go there!!" Apparently the work already done on the O/S rear wheel arch on my car, would in his opinion, be due to rust at the interface between bumper and metal mudguard/wheel arch. I am now sure that the problem is caused by chafing of the plastic against the metal, and could have been avoided by fitting a rubbing strip between the two. Looks as though the bean counters at GM have been having their wicked way again. Anyway the cost, which includes cleaning up both wheel arches, as the areas around the door openings need doing as well, and a small area at the bottom R/H corner of the boot lid, comes to £250. No more ice cream, chocolate, meat, fish, or chicken, next month then.
Best wishes, Jim.