Wall mounted mixer tap?

I have a Gessi oxygned tap that when turned on leaks underneath the handle,I think the cartridge has worn out.I've removed the handle and now need too remove the ring nut with the key/socket that came with it but as it's 5 years old I cant find it.Is there any thing else I could use or can I buy this "special tool" anywhere?
Any other info greatly received.
Cheers