plumbing boiler ariston 27 Mffi?

I have a problem with the HWS with the boiler, the flat has been empty for 2 years and did not know if boiler was working before. The central heating works fine. The micro switch 'LED' on the hws brass flow switch does not light up until you pull this switch half way off the brass flow switch (water side) the LED lights up and boiler then fires up but it stays on even if the tap is turned off, as soon as you put switch back into its original position (cliped onto brass flow switch) fails to work. I have taken hws flow switch (water side and cleaned). It seems to me to be the LED micro switch. Can anyone confirm or ?