I have done this and all that is happening is new parts are fitted and the boiler is just the same. I need to know what's wrong or could be and look at my options.
You should not have to pay for parts which are not needed to repair the system.
Nor in my view should you have to pay for the diagnosis either as that has not correctly identified the problem.
Some of my industry colleagues dont agree with that as they argue that they have spent their time and they should be paid ( even if they have not managed to find the fault ).
I would reply that they should not be taking on work that they cannot do. I would never charge if I was not able to find a fault.
Tony