Thanks a lot Steelmasons. I knew there would be something I had missed (and possibly some more). Unfortunately I am away the next few days but will try it when I get back.
My lack of experience led me to the belief that if the thermocouple kept the pilot going it would have enough power to work that solenoid, but of course not necessarily....
Now what else? As the pump doesn't seem to have sufficient power to work the CH switch servo, I have two more pumps (one new) and might try that. It has a slightly lower wattage but is in better nick of course.
The wiring harness works but is pretty sad.
As to some of those other comments, I always give suppliers back things I cannot use (for whatever reason) unless it is impossible. We're not all necessarily grasping everything we can lay hands on.