(http://i1069.photobucket.com/albums/u472/carlgordon75/028ad7b4.jpg)
It looks really rather bent in that photo and does look like it's the reason your wheel cover will not fit.
Yes it is bent - I have managed to straighten one in this condition by heating it with an oxy/acetylene burning torch then bending it back with a bar of similar diameter to the front fork fixing hole - However it is hit and miss - if not you are really looking at a chassis swap.
There has been some welding done there and it is distorted.
You have the option of running it "as is" , it seems nicely done so probably plenty strong enough , or go with a chassis exchange.
If you're not up to doing a chassis job yourself bring it down and I'll do it for you. Email me if you want to look into this option a bit further :).
Thanks guys, I have completely stripped the c5 now, got a welder loking at it tomorrow so hopefully he can sort it!