£30 or so to modify the head whilst its off and at the engineers.... versus getting some plugs imported from the states

Ello Phil, alright?

As for Focus RS plugs - not sure about that as a solution. I think Paul J used them and had a tip fall off, but im not 100% sure on that. Im fairly certain A plug tip melted on his, and did some damage to the head / valves or something, so worth doing the cossie plug mod IMO!