I know this is a really old topic but sees as I do this stuff as a side job I couldn't help replying

Garages will rip you off bud and more often than not the workmanship is very poor like spot welding or blob welding or just putting plates over rust patches, you could do with finding a private welder with a small workshop like my self, the fact that I can see an outer sill that's only been riveted on makes me think how far this rust as traveled, I think the mot states now that new sills need to be seam welded right through, I charge only for materials used / how many hours it takes me to do this job, you have quit extensive rust there what makes it worse is it's around the seat belt anchorage a common mot fail on early fords, it;s so common I have steel that I ready made / shaped on the shelf which I weld straight in lol, I've lost count of the number of fiestas and Ka's I've had in with this problem, I've just finished a Mrk4 fiesta which was so badly rusted at the rear wheel arches that the rear trailing arm had pulled away from the frame on the near side, never seen rust so bad, I cut much the inner wheel arch away and where the trailing arm mounts to the frame to make way for new steel and welding new box work for the trailing arm to bolt onto, total cost £150 5 hour work which includes materials and wax oiling everything, the rest of the car was sound even the sills so it was worth saving.

What ever you have done there will need to be seam welded because of where the rust is and how bad it is, anything that effect structural strength needs seam welding, inner and outer sills, suspension struts, floor pans, usually I charge £50 for sills as it's an easy job, I don't bother patching I prefer to cut the old one out and weld in a new one