Okay, forget my previous posts. I've got a new idea below:
(Size is off completely, just for illustration purposes.)
It consists of a waterproof solenoid valve:
A BSP threaded barrel nipple cut in half for the two ends
And some thick clamps for the joins
All to go on this section of my bypass:
Nice and simple. I'll probably fashion a heat shield for the solenoid itself, but should be okay. Any issues anyone can see? Can't see why this wouldn't work.
Also, apologies for the deleted posts - if an admin could remove them, that would be grand.