Very often it is not the boiler that is at fault, but an auxiliary part of the rest of the system.
It is important to be able to quickly diagnose whether it is the boiler or the system which is causing the problem. I have many years of experience and will not waste time looking for a boiler fault when it is in the system or vice versa.
Sometimes a fault might develop in the controls, such as a thermostat, time clock, diverter valve actuator etc. I am fully conversant with systems, their wiring and their controls. I do all my own electrical wiring and fault finding.
Also, I am qualified and experienced with unvented Domestic Hot Water cylinders and thermal stores. Unvented DHW cylinders have to be maintained and repaired by a qualified and experienced engineer, otherwise they may become dangerous. They are of course wonderful when they are maintained correctly. Thermal stores are losing their popularity nowadays with the advent of unvented cylinders, but there are still a lot out there and occasionally they need repairing.