Combi intermittent hot water

[ocemeer]

My Valiant combi boiler is not kicking in when hot water is being demanded from the kitchen mixer tap. It’s working perfectly okay when using the bath taps upstairs. There has been no reduction in flow from the kitchen taps; what is likely to be causing this.
It is booked in for a 12 month service will this be an expensive fix?

Thanks

 

[GasMan1965]

Have you got a model number ?

Could be a flow switch or diverter valve !!