Intermittent faults are always difficult. If it won't go wrong in front of him, he may not be very sure what's failed. If he puts new parts in and it doesn't fix the boiler, do you expect him to remove them and not charge you anything?
The rrp for the sensors he replaced is over £30 + vat, each.
If his experience tells him they're a likely component, then it's reasonable to change them, isn't it? They often fail in a way you can't easily test for.
If he tells you the sensor resistances had drifted from the new values, surely he's justified in changing them?
How long did he test the boiler for? If it was more than the period the boiler usually fails in, what's he done wrong? Boiler fixing is not always an absolute - when they're old, nothing works like new, things sludge up, and so on. It can be impossible to tell what has been the "last straw".
If he waited 10 minutes and it was OK, and it sometimes only fails after an hour, what do you expect him to do?
There are those here who will tell you that they can ALWAYS completely fault find a boiler at one visit. They aren't necessarily telling the truth! I certainly can't guarantee to do that. Like cars, they can muck you about.
You pay for the guy's time, judgement and best efforts. If it doesn't yield the result you want, franky it can in all fairness be just tough. If the bolier had better diagnostics it would help- there's practically nothing on that boiler. If your car goes wrong or you see your doctor to fix something and it doesn't work, do you get your money back? NEVER! There is no NHS for boilers!
Yes he should stop the leak.
If he DOES see the fault occuring then he should be able to eliminate a few possible causes. In my experience of these boilers. intermittent faults often lead to the pcb needing replacement - very expensive!