Not trying to sound like a twit but that does contradict your earlier statement's meaning a little. The engineer did fix the problem, but at the same time the customer was overcharged by replacing a part that did not need replacing. It's a moot point and you get what you get on the day.....
Charlie
Not really. Crap in system causes blocked plate hex.
The hex is part of the boiler.
Replacing/cleaning the hex will solve the problem, and as such a valid repair that will get your boiler working there and then, which is what the engineer is there for.
Leaving the crap in the system after the repair, is like repairing a puncture without removing the nails on your drive. You can't blame the garage for charging you for repairing/replacing the tyre needlessly, nor can you expect them to clean your drive for you.