yellow for gas blue for water
sounds like its the cold mains feed into the boiler.
are you sure its actually dripping from this pipe as often the drip can be coming from higher up in the boiler and the drip then falls onto something lower down giving the impression its coming from there.
if it is coming from the pipe you suspect then i would hazard a guess that they have turned off the cold mains feed into the boiler to make the repair because those valves are prone to leaking once they have been used and turned back on. one option is to replace the valve the other is to wait and they usually seal themselves up after a while.
if the waters slow then its probably down to a flow restrictor in the new flow switch, this may be adjustable.
call your service providers back out and tell them your not happy.
everyone should be aware that these boilers are very complicated machines and not every engineer can be an expert on every different make and model and we cannot always fix a problem first time.