Long time since I have tried this but I remember that rotary stones are no good because they clog up but I never really found a suitable cutter either. Ended up doing a lot of work with hand tools.
What is recommended for use with a drill and flexy drive and where do I find it?

When I did it I used some Carbide BURR from EBLAG on a DIE grinder (air)

linky

As posted by mad4x4.....This type of thing.
All shapes and sizes available. Lots of revs if possible and WD40 to speed things along.
Geoff.

If you haven't got a big compressor for the air die grinder, try this kit (with the carbide burrs as above)

Rotary tool thingy

That Clarke kit looks like excellent value despite only having 1/8" shank capacity.
Might get one myself.

Geoff.

i done mine with normal drill, £10 flexi drive and a round ended rotary Burr and a couple of flap wheels. i have a air die grinder but didnt need it flexi drive was ok, good to get one with grease nipples and keep it grease

Keep a bar of hand soap on the bench and run the carbide burr through it once in a while, helps keep the tool from clogging up.

Carbide bur is the tool but liable to clogging up. I'm told that tallow is the thing to use but I found LM grease worked ok.