Hi it’s a standard (Toby ) that’s what it’s called in the west of Scotland, if you have a standard water key it will fit on to the flat bit which you need to turn a quarter turn, if you look just now the flat bit is in line with the pipe below
turn it a quarter turn so it is across the pipe
it usually doesn’t mater if you turn left or right but turn it clockwise , some time they are very tight but it will go.