Chris,
I puzzled over the empty / unmarked area as well, but given the logical locations of 792, 801 and 815, thought the one my picture was the most likely. I did stumble on a stray kerbstone in the empty plot in the row above Arman's but can't for the life of me remember what number was on it. If you are up there again you will see where I cleared it off. Unfortunately I was tight for time on Sunday.
I find that, generally,Greenwich is one of the best for logical numbering, but, I suppose that only the Cemetery Office will be able to confirm the location beyond doubt.
Phil