I finally manage to refit the outer stills on my 1997 BMW Z3. These sills are non-structural on a Z3 and so they do suffer from rust and corrosion but can be bought, painted and refitted which renews them and helps protect the car.
I first undersealed and solved a bit of surface rust on the inner sills/chassis and then refitted my new outer sills, along with the awkward mounting points at the front and rear!