Thank you - that's really helpful.
The rust is minor, mostly surface with the occasional hole. The biggest problem is a series of holes along the flat surface around the fusebox. It's just rusted through and was concealed by the sealant. I only noticed it because I was wondering where the water was coming into the driver's footwell.
Since the photo, I've been a bit ruthless with the metal, grinding out the majority of the mess and intend to plate it from inside so that I can put a skim of filler on top and make it look original. So I wanted to eliminate any chances of it coming back and at the same time treat some other areas.
The heater cowling is rusted away in one point too, so I will try to repair that with some fresh metal.
Thanks again