Yes, that's right, Paul. There is a hole in the top of the valve spindle, The extension rod fits snuggly into that but both are circular so there's no way it can engage and open/close the valve. There is about 3/16" of the valve spindle proud of the valve mounting boss. It's not a lot but it may be enough to drill a hole through for a locking pin or even a slot.
I am hoping the supplier will get back to me on Monday with their recommended method.
Thanks for the reply,
Michael