jacko wrote:What is classed as a cover sill ???
Originally a cover sill was as it sounds - intended to be welded over an original sill. Welded properly it would probably bring back enough strength to be as good as the original, but often these weren't welded on as fully as they should have been. Usually (though probably not in the case of Ex-P doing new ones) they were thinner metal. Crucially, as they were designed to fit over the original sill, they're not the same curve, they're a little bit bigger. Same as wheelarches, which have a different curve so they fit around the existing panel. Probably not noticeable.
The top edge is the bit I'd probably struggle with making - the bottom flange and the curved section is no problem. Remember as I said earlier, the sill itself is only the bit below the door - if it's gone further back, that's the bottom of the rear quarter panel and there should be a visible seam between the two. Whether you go to that trouble is your choice - and I'd imagine will depend on whether the seam is visible on the other side. While the sill is off, there should be a closing panel towards the back, closing the inner section of the sill.