All the above offers and links are very much appreciated. Thank you!
I should explain that I am in a bit of a Plumbing Phase at the moment. The system that was installed in our house has the drain cocks in some very strange and inaccessible locations. (As it was was installed by professionals, perhaps speed was of the essence.

)
The drain fittings have a 7mm square to operate, and the normal sort of key is very difficult to fit. It could be done by the usual adjustable spanner or even slip-joint grips (if a proper plumber was doing it

), but I felt there must be an easier way, viz a small socket on the 1/4" drive snappy ratchet.
The pics show square-drive keys, (the two smaller (5mm) ones are for radiator bleed valves), a drain-down fitting, with lastly my planned solution.
Pete