Richard's supplier is
http://www.rollcentre.com/ If Richard doesn't want to pursue it, you can try to contact them directly. Custom-cages do an S1 rollcage, but they aren't very certain of their measurements, so they will prefer it if they can install it and they are quite expensive.
If Richard's cage turns out to be a dead end, I would suggest you contact a rollcage fitter near you, you have quite a lot of choice in the UK, and you can find some reviews of any fitters online.
I might do my own S1 cage, but I'm not quite sure yet. The fitters in Belgium and the Netherlands are quite expensive and they don't know all of the caveats for Europa roll cages.
Serge