I thought for a while (novel concept), and devised an alternative - and because it seems to have worked I have detailed it below for comment. Clearly if it had been a right mess I would have kept quiet, bought another sill and opted for either of the first two....
Anyway. Step one use a dremel (or cheapo equivalent like mine) with a small cutting disc to make three small incisions like so.
This allows you to get an air-driven saw into the slot and cut out three sides of the recess - my cheapo dremel/saw is not up to cutting all three sides without a long lie down.
Push the 'flap' out from inside so you can use a grinder to remove a mm or so from the long edge and a triangular wedge off the sides (more towards the long edge) such that when you bash it flat the bits don't overlap - easier than perhaps it sounds
Using panel hammers (and I used a piece of 1" round bar welded to a stake stuck in a vice - it's in the front of the pic below) to flatten off the whole area.
Run a sanding disc over it to check for highs and lows - tap out accordingly
An hour later - voila (better than drilling spot welds)
All I need now is to find a way to make exact 8.5mm and 15.5mm metal circular discs to fit in the holes ....... then TIG weld and prime.
spent 8 months part time welding on mine.
)
