HI
You want a long box spanner or the next size down long reach socket
use one of the other plugs and find a socket thats just smaller and then knock it onto the damaged plug, it should grip it enough to get it out
or even a piece of steel pipe, you want it, just smaller than the plug, then knock it on, if its one of the back ones, you will struggle
dont use a socket with the 12 flats, use one with the 6 flats
if you can find any thing, get some steel wire 1.5mm to 2.5mm, like a welding rod, bend it into a shape similar to the spark plug, you want it so it slides inside the spark plug socket, so it slots into two opposite corners, then at the bottom of the socket, bend the wire out so it can't slide into the socket
you want this wire to take up the slack from the corroded plug and socket, you want it a tight fit, so it knocks on, light taps till it goes on, the tapping will also help free the plug
hopefully that should get it out
also if the car is still a runner, the plug insulator is not damaged, then I would only try knocking the steel pipe on it or a smaller long reach socket, as any thing else might damage the insulator, leaving the plug faulty and then a non runner
cheers
Joe
sorry for the quick sketch, but it should give you the idea