Valliant Combi Boiler Hot water

I have a 10 year old combi boiler, I have had no issues with hot water until I changed my kitchen tap. The water flow rate seems the same as before but the combi boiler does not switch on, unless I open another hot water tap in house, and if this tap is then closed while the kitchen tap remains open the boiler stays on and supplies hot water.

Flow switch on the boiler has probably become a bit marginal with age, so requires a greater flow of water to move it. Once it's moved, the lesser flow from the kitchen tap alone is enough to keep it operated.

Boiler needs a new part - not a big deal but an engineer will do it better / faster then you can.

Ten year old vailants hot water work initially by the movement of a rubber diaphram ( which when worn give your symtons) ,which is fitted in the water section located in the centre of the boiler at the very bottom about 3 inches round with four pipes going to it and held in position with grub screw top centre. There is also a small pin in there that scales up and stops full movement. Not really a diy job but should only take a pro about half hour with part being no more than £20 as opposed to new water section at approx £120

Thank you for the advice