Well I got the underside clean enough to apply rust killing paint today
The pain in the backside parts are things like the trailing arm mounts and the jacking points - very difficult to clean out the gunk / rust / crud from these.
Now this paint (looks almost like the same colour as the top!) is the Hammerite No 1 rust beater stuff that I know is pretty good at doing what it says on the tin
Most of this underside is going to be painted in Eastwood extreme chassis black - as it is a 1K paint I'm certain the combination of these paints will be fine.
The sills and the spare wheel well are going to be painted in 2K body colour. That might prove to be more problematic - I'm not sure yet if the Epifanes 2K epoxy on top of the Hammerite is going to work...