Well in the course of fixing my C5 up I've ended up with a large bearing out of my gearbox left over (as I bought a new one, the old was rather rusty and useless), I've jammed that over the bit that sticks out on the left (outer) of my drive gear and it's holding it together wonderfully. However, it's still cracked and a bit broken, so I'm still looking for a replacement...
I've sent some enquiries to a couple of companies, I'll see if I get any responses...