Worcester Highflow 3.5 - Slow Response to Hot Water Draw

[PhilWorcester]

I have an 18 year old Worcester Highflow 3.5 - operating in a soft-water area with no scale. On drawing hot water, an initial pulse of hot water is followed by a period of cold water - then reverting to hot water again. This appears due to a certain degree of tardiness on behalf of the hot water sensor - the gas valve and diverter valve take multiple seconds before responding simultaneously to the hot water draw. Over the years, the pump, diverter valve and hot water inlet sensor have all be replaced in a bid to address this (and other) problems. Presumably this slow response of the inlet thermostat is a well-know issue with these boilers? But is there an underlying cause and what is the fix?