I can't say what parts are needed, fault codes are just an indication, the actual failed part could be many things, possibly flue fan.
some plumbers are lazy and are not prepared to diagnose a fault, some don't have a clue how to diagnose.a fault and just replace parts willy-nilly, or the whole boiler
combi boilers have many electrical safety systems, making them very safe. but complex. more of a job for an electrical engineer or fully trained technician.
A good plumber/technician would probably have diagnosed the fault ordered the part, a second visit to fit and test, probably £300
a new boiler is like buying a new car, some can be had with flue kit for around 500 .others would be treble that.
fitting (including commissioning, testing, certification, flushing, etc etc )maybe 500 quid
but in areas like London, with parking, congestion charge etc, prices vastly increase