Thanks for coming back to me on this.
The valves are these ones:
I have attached an image showing one of the valves wired up. The live (brown) wire goes to the 24v terminal on the valve and back to one of the switch ports on the OSPi
The blue wire goes to the COM port on the OSPi.
I’ll test the voltage later on today and update here (although when you switch off the OSPi, the valve also shuts off – so I imagine I will find that voltage is still being supplied)