I was lucky enough to find a 1", 3/8" drive socket at the local flea market. With a bit of grinding to the back edge I was able to feed it in as he says, from behind and turn the corner up to the valve. Only then can one fit a universal-joint adapter and long extension to the socket. Very fiddly but it can be done.
One can probably use a 15/16" socket to install the valve, but you're likely to come away with only the top of the valve when using one to remove it. Ask me how I know this.