Not Staffs, but Nuneaton; MNS Accident Repair, round the back from Sam Goodwin. Martin has done a fair few MX5 sills (including mine), and knows how much needs to be chopped out. Some body shops I spoke to hadn't a clue; one tried to convince me that he only needed to cut out a circle and new metal welded in.
Not sure what he will charge just for a sill repair, since he welded my sill up as part of a respray (£1200 for sill, dents, carbon fiber bumper restoration and full repaint). A year later, when the other sill started to show signs (water bleeding through the paint), he did that sill for me FOC.