Sounds like it's a mo-mo valve - so it needs power to open, then also power on another terminal to close. No spring loading??The VC 4613 is a balanced valve. not you standard S plan.
So the cylinder stat needs 2 wires to the valve, from terminals 1 and 2.
If you've got a mo-mo valve, timer sends power to top stat - C. If stat needs heat, it sends power from term1 to brown 'open' side of valve, if it doesn't need heat, it must send power to black 'close' side of the valve, off terminal 2 of the stat.
If there is an orange wire from the mo-mo valve, does it go live when the valve opens? If so, power the boiler from that. If there's no microswitch on the valve, and the dead orange wire is just there to 'test' you, then you could hook the boiler to terminal 1 of the cylinder stat - if timer is on AND stat calls, boiler is fired. There's no failsafe there though - if the valve failed to open, the boiler would still come on.
Maybe? I'm not an expert
