Rover streetwise idling too fast

Following a head gasket change and much connector disturbance, my car idles hot at about 1500rpm. I have checked all connections, and swopped the idle control valve for a second hand one. If I run the engine at idle and put my hand over the idle control valve input hose the engine stops, suggesting no other leaks. My impression is that the valve does not close down when engine is hot. Any ideas? Engine ECU temp sensor? 2 duff idle valves?

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