Why does my boiler come on an hour or more after the timer control has turned off?

When the boiler fires, the radiators do not get warm, but I don't know if the hot water is being heated. The controller is completely off, and the roomstat is set to its lowest temp. The cylinder stat is turned right down - 50 degrees. The motorised valve was changed recently, and it may be this strange phenomenon happened after that. The new one is a Honeywell. Could the wiring be causing the problem? The only way I can turn the boiler off is by turning off the electricity to the pump.
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