So here is an update on the brake caliper refurbishment. After being in the electrolytic tank for a day or two, the caliper has been washed in water and wire brushed. Almost all the rust has gone. You can even read the makers name on the casting!
The electrolytic tank is a plastic container with water mixed with washing soda (£1 from Morrisons). Then attach a piece of scrap steel to the positive terminal of the battery charger, and the negative terminal goes on the part to be treated. There is a lot of info online on this technique, some use a more dangerous mix, but I find washing soda works well, and is relatively safe on your hands. It takes a few hours to get the rust off, and you are left with a nice smooth surface.
Next job is to fit a seal kit and then maybe paint the caliper to slow down the return of rust.