I'm being charged for Boiler Diagnosis and wasn't told?

You're basically paying for his time. He gets paid by the hours he's in work. If I got a plumber to diagnose the problem and it wasn't anything to do with the work he done, I'd expect to be charged for the call out. I'd of offered him the money on the day for the call out to be honest and went from there. I wouldn't expect any tradesman to turn up for free unless I've stated exactly what I wanted and just needed a quote.

It is reasonable that he has charged you for diagnosing the problem. He has overheads to pay such as fuel, parking and upkeep of his vehicle etc. You are also paying for his time to attend.

I would say £40-£50 would be reasonable for call out and diagnose boiler fault . I wouldn’t charge any of my customers any more than this

yes he needs to get paid for the call out