Even by just posting on here you are giving the impression that you are expecting a free diagnosis by someone who has never seen your boiler.
The only way that a boiler fault can be diagnosed is from in front of the boiler. [ From where some simple preliminary tests can be done. ]
Any competent boiler engineer would want to make a diagnostic charge for the diagnosis. Most would expect to charge at least £84 plus any parking charges.
Most boiler engineers will be happy to discuss their charging policy before visiting. I always diagnose faults at a fixed fee. It seems wrong to me to charge by the hour so that someone who is very slow will charge more. I usually expect to diagnose most faults within 20 minutes. But two took me about three hours. One because I had made a mistake with my method for doing a simple test.
Most would expect to correctly diagnose the fault, charge for the diagnosis, and quote to complete the repairs needed.
Some are not so good at diagnosis as that needs a higher level of competence and experience. Hopefully anyone who fails to correctly diagnose the fault would not expect to make any more than a most basic charge to cover the costs of visiting, perhaps just £36 for example.
You seem to expect that your boiler only has a very minor fault. Many people have that false expectation that their boiler, car or TV only has a minor ( and cheap to repair ) fault. In the real world that is not always the case.
The fault could even be with the gas supply and nothing to do with the boiler itself !
Many boiler manufacturers have their own team of engineers and many do a fixed price repair. But they are mostly about £300 or more now so not so good if it is really only a minor fault.
To make you laugh I will tell you about an experience which I had which made me laugh. A solicitor's wife called me to their boiler on the third floor. I identified the gas supply was seriously impaired and asked her if there was any emergency gas valve in her flat. She was quite sure there was not.
The meter was outside and 3m from the building with the pipe going under the garden. I explained to her that I would need to inspect the underground pipe.
Her husband then spent an hour or more digging up the pipe as a £15 an hour job instead of his usual £300 an hour solicitor job.
I returned and checked the underground pipe which was OK. Gong back into his flat I traced the boiler gas supply pipe around the kitchen. It ran through a cupboard stuffed full of plastic bags. Removing the bags I found the emergency control valve which had over time been moved downwards and had almost completely turned off !
It certainly made me laugh, Tony.
You got the guy to dig up his pipe, so you could “look at it”?
For £84 I would have expected YOU to check for the required ECV, and test pressures.
What exactly did you want to look at?