How can I undo the locknut on this Shimano 105 hub I'm having trouble working out how to properly service this Shimano 105 rear hub. I've watched some videos, read Leonard Zinn's book and following Park Tools' instructions, but it doesn't help.

I've got a 15mm cone spanner around the cone (I think that's the part in the picture), but there are no flats of any size on the locknut.

I got some vise-grip pliers and managed to get it undone, but there must be an easier way!
OK, I eventually worked out the easier way.

The locknut has a hole in the end which, in addition to taking the skewer when fitted, has a hex socket for a 5mm allen key. Turn the hex/allen key anti-clockwise relative to the cone spanner to undo.