A bit more to add to the story.
My gunsmith headspaced it quite tightly, to try and take away some of the usual dimensional variances of the belts on belted magnums, which will most likely be a benefit to accuracy, although I'll make it headspace on the shoulder.
In doing so, an interesting point is noted.
Originally I necked down 100 Hornady 458 cases to 416, after loading it is very plain to see that the dimensions of the belts vary greatly with Hornady brass. I only necked the 458 cases down far enough to get the bolt to close with a little bit of resistance. Some of the cases, about 50% are really tight to chamber, I ran a black felt pen around the belt area and the shoulder area, chambered a few of the tight cases and lo and behold, the front edge of the belt had the pen removed. Winchester brass is a lot better than the Hornady in this respect, although there were still a few tight ones.
Goes to show that paying a lot for better cases is not necessarily a good option.
Also goes to show just how sloppy factory chambers and therefore headspace can be.
Our solution will begrudgingly be to take the chamber in about half to one thou to get the tighter cases to chamber.

Anyone else had similar problems, not necessarily with Hornady brass, but any brand of brass?

David.