Hex key size for radiator valve extension?

[Plot1]

I planning to order some of these radiator valve extensions.

If you've used one before, what size hex key do they take? I'd like to order that online too but ToolStation didn't have the information when I called them...

 

[Boilerman2]

Buy a Radiator Tool with your extensions, this will have the right size hex it not sure of actual size

 

[Boilerman2]

Toolstation part No 79854 £18-25

 

[longdogs]

Use plenty of ptfe tape around the threads otherwise they WILL leak

 

[doitall]

12mm socket about £6 quid