Ariston combi not modulating

[Wyedean]

I have an Ariston Micro Genus 27 combi which operates ok in heating mode, but only minimum burner on hot water demand. I can manually adjust the gas valve to give a higher burner rate on hot water, but it won't modulate down when the flow is reduced (so water temperature rises dramatically).
I have spoken to Ariston technical help, who advise that the gas valve is defective; but before I rush out and buy one (£100ish), have you any other ideas. Might it be a control board fault. Can anyone tell me what voltage/current I should getting on the solenoids? Ariston tech help weren't very helpful with this.
The gas valve is a SIT 845 SIGMA.

 

[Puddle]

Well now you have increased the gas pressures on hot water mode it will not modulate down to low flame as it is the low flame you have increased

Put gas pressures back and first thing to check would be the hot water probe/themistor as this probably faulty

Remember you should be competant and gas safe ect to work on gas side of boiler