Theres nothing the sparks couldve done to cause the problem, unless he took the cover off and messed around the internals. Its a coincidence.
Have you got the possibility to measure how many liters per minute of hot water are you getting at each tap, including shower head? Unlikely you have a measuring cup, perhaps get a 10L bucket and see if it fills up within 1 minute, you should be getting at least 7-8 liters at every tap. Unlikely you have a restriction, but worth checking. If you have less, see if the cold and hot valves under the boiler are fully open. Also check ch pressure, if too low it could be on the brink of shutting down, but opening 2 taps makes it come on (i know it doesnt make much sense but old boilers are very responsive to stupid ideas that wont work on new ones). If these check out, the problem is on the inside. Careful, not every gas safe eng knows how to troubleshoot when dealing with breakdowns, I swear 99% will just throw new parts at it until the problem goes away, and even then some couldnt say what was it that they just fixed. The component you need (if you need it), will cost pennies compared to usual boiler component prices. Do not get fooled into changing the pcb or diverter valve, problem is not those.
