Alans comment on this turned out to me quite accurate
Today I finally got a chance to look into this. I thought I'd carefully cut a piece from the new 1/4 panel, using a spot weld cutter to minimise the damage and let the new piece into the bottom of the B pillar
Spot welds cut out
Piece removed
And a perfect fit
Sadly as I cut back I found all was not well. This is really annoying as it's the only replacement panel on the car and it's let the moisture in to attack the sill - forward of the B pillar the sill is as solid as you like.
Here you can see the bottom of the B pillar (solid thankfully) and how the 1/4 panel has been layered on top of part of another, whether the original Spider panel or what was left of the coupe base from the original build I don't know
So....
Clearly the sill needs repair and I'll need access to get to it.
I could cut the panel to the along the lower swage line as far as the wheelarch, repair the sill and then make good or I could remove the whole 1/4 panel, repair the sill and put the new one on. Paint will be needed either way.
I'm unsure which to go for at the moment - they both have their problems and merits.
I should point out the rear outer wheelarch needs some bubbling sorted and the alignment of the panel isn't 100% at the rear panel / boot join.