I recently had an annual service on my boiler - prior to this it was working fine. After the service the pilot light would not light. Another engineer cleaned it and it worked ok for 4 days. Today the boiler seems to be overheating - after 30 mins a click is heard and the pilot light is extinghuised which can only be relit after the overheat reset button is pressed. The engineer today has told me that this is nothing to do with service but because there is sludge in my system which needs to be power flushed. My boiler is still only working for 30 mins at a time. Can anyone advise if this solution they are telling me is correct as seems a bit of a coincidence that since serviced now not working properly?