Tried this today, left it bubbling over night, washed and brushed off loosened rust with a nail brush. Immediately dried it and put it in the Rayburn oven for a bit, then a quick wire brush. worked a treat, used battery charger, 6v trickle, 12v bulb (for safety, it just glowed red) in line to reduce amps and lessen scumming.
Why I didn't try this years ago? It's brilliant, read on the web that someone had done a chassis in his swimming pool!
http://myweb.tiscali.co.uk/andyspatch/rust.htm