Carbide Die Grinder is the official name.
http://www.ebay.co.uk/itm/5pcs-12mmX...4AAOSwDNdV5Xs8
The one I used was called a metal muncher and looks similar to the one on the far right of that ebay advert.
I then used a fine pointed one to finish. But 95% of the work was done my the muncher.
I was very surprised at how quick it removed material compared to the B&Q regular steel one I tried beforehand