Calorlite pigtail fitting

Qashqai66

I always seem to have trouble with the pigtail fitting for Calorlite bottles.  The gas spanner I have is one of those cheap thin flat ones which is not even a proper fit for the nut on the pigtail connector.  The result is that it slips and I end up with
bleeding knuckles!  The design of the handles on the Calorite does not help as they reduce the amount of spanner movement.  Does anyone know the correct size of an open end spanner which fits the nut on the pigtail?  I do not have one large enough in my spanner
collection so need to buy one.