Ariston DIA 20 Boiler -- slow to start heating water

Our DIA 20 combi boiler, about 10 years old, has become slow to detect mains water flow, ie we run the "hot" tap for ages before boiler activates. Then, once it has primed, it works OK. This response delay has been building up over the weeks. If tap is not used again for a while, same slowness thing occurs.
The filling pressure from the filling loop is OK, and CH section works when asked for heat.
Is there such a device as a flow detection valve that needs replacing?
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