tube spanner
Although a tube spanner works fine by feeding the cable through it, this is the other way that I didn't tell you Paul.
This way works a treat, one of the best tools in my box.
Saves pulling the seat and tank off too
Depending upon lever type? you can just undo it's bar clamp so the lever assy is free to roll and rotate with the cable, assuming that you only use loose cables ties for keeping the cable to your frame backbone and not gaffer tape crap.
Buy one of these 21mm 13/16 el-cheapo plug sockets with an outside hex on top, mine came out of one of those $9 reject shop socket sets.
This is the other side of the socket.
Clamp it in the vice and cut a piece out with your A/G slitting blade to form a slot so it slips over and back off the cable
Use your open ender spanner on the end hex. Remember these valves are only tensioned to 18 ft/lbs(25Nm)
The socket I used, has only a six side internal Hex, which is my preferred.