Give it some thought before trying, but i laid mine on the floor, held it down with my hand and with the fill nipple at a 90 degree angle to the floor, gave the nipple a tap with the hammer. This is not a full on hard as you can whack, just a bit of a tap and found this was enough to loosen it and then unscrewed it by hand.
Also bear in mind I had no intention of ever reusing the reg (although i don't think its damaged in any way from it)