I haven't worked with oil in a while now and my OFTEC has expired but when I did do that work I occasionally had a complaint that I was "taking too long, the last guy used to do it in 20 mins!"
An average oil boiler service in my area would cost £100 with an oil filter change and new burner flexis. It would take me approximately an hour to do it properly if the boiler wasn't too filthy. One day a customer complained to my boss that "the last guy only took 20 minutes and only charged £80."
All they were doing was changing the nozzle and checking the gas readings. Nothing else, no cleaning, no testing safety components. Just nozzle, air adjustments and leave.
He pointed out that nobody could service an oil boiler properly in 20 minutes and would they rather pay £80 to not have their boiler actually serviced or £100 to have someone spend an hour and do it properly? Thankfully they stopped moaning after that.
You get what you pay for in life usually.