I wouldn't be questioning the price, but more the time allocated.
10 hours total for seem to be very easy faults to diagnose.
Without being on the job I could be wrong yes, but most faults contained within the boiler can be diagnosed within 30 minutes, especially fan and pressure switch issues.
Let's say he took 30 mins to diagnose each fault, that's 2 hours. Then we add an hour and a half total for fitting which takes us to 3.5 hours. Include any travel etc to merchants and I still struggle to see how we get above 6 hours total which gives a total of £270 labour, not the £450 you were charged.
To give you an idea of duration, I recently on a job removed lead from a gas meter and replaced with a new union, ran a gas pipe to the new utility room which was 5 metres away, installed a new combi boiler, installed a magnaclean filter, hung 6 radiators, installed a hive thermostat, filled balanced and vented system, all within 10 hours which included trips to the plumbers merchants and a lunch.
I would see he has overcharged, but without seeing the work and knowing the faults, don't quote me.