Following the construction of the folder, and making up the sill strengthener,
I offered it up for the sill.

Just held in with mole grips, as is the b/c post.
As an indication of the strength of this panel, after I had tack welded the A post end
it was very hard to move the b/c post end at all.
Once I was happy with the position the panel was seam welded along the bottom and plug welded at the top to retain the stock look.

Ok so with the B/C post removed a new strengthener section was required a) to add some additional structural strength to the sill, and b) to fill the gap created by moving the B/C post back. But first what I needed was a folder capable of bending roughly 3 ft of 18 guage steel. I had some steel box section, so I bought some angle iron from B&Q along with some threaded rod and some nuts.....
 
Once the folder was complete I made the sill strengthener section.....

Moving the B/C post was the scariest thing for me so far, but turned out to be quite straight forward. Basically it involves drilling out all the spot welds around the bottom of the post, cutting the top of the post near the roof, slinding the post along the sill to the desired position, and plug welding the drilled spot welds.

As you can see from the third picture, the outer sill mounting panel doesn't continue through the B/C post but stops and then restarts. That was fine for me as I am replacing the front door section with the sill strengthener.