Silly me.
My non-functioning solenoid valve has red and yellow wires; red and yellow wires at control board go to a relay. BUT, solenoid wires become red and blue somewhere in transit, and one of these goes to a tiny surface mount triac (at least, it looks like a triac footprint) which carries the designations 29M and F933.
Is there even any point my trying to solder on a replacement? I had to fire up the USB microscope even to read the numbers. I have a small thermostatically controlled iron, desoldering braid and sucker.
Having ordered a relay, I'm half tempted to try to drive it using the signal from the faulty triac.