This is a known fault with the CP diverter valve assembly, need to replace the complete assembly (Head, Valve and Manifold/body) for the latest complete Honeywell valve/head/body assembly.
Glad you finally got it sorted Tombarry, persistence pays off in the end eh!
Thanks for the update massgsm, really helpful. I think I'll stick it out until the spring and then get it repaired under contract, but maybe its time to start saving up for a replacement.............
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