Hi Richard,
I'd be careful before you shell out 70 quid on this. If you're plating onto clean-ish bare steel I'm sure the kit would work fine. Once rust has got a hold and etched into the surface, it would need, at the very least, pretty vicious chemical pickling before plating. The passivation process needs to be done after plating as it seals the zinc and gives it better corrosion resistance.
If you have a word with a local plater from your Yellow Pages, I'm sure they'd tell you what can and can't be plated...and explain it better than me!
Cheers,