It is a 30 CTI. The heating switches off when the hot water is run, we have had the alternating valve changed but that didnt help (that may not be the right word - is it a diverter valve?) Then the F6 fault comes up and there is also a warning light flashing on the wireless thermostat. Nobody has been able to fix it!
When you use different terms in the same posting it makes me wonder if you know the difference.
A plumber primarily specialises in plumbing (!) but can also be gas registered and do some heating as well.
A boiler engineer is someone who mostly exclusively deals with boiler repairs and intimately understands how they operate and the properties of the components involved like the sensors.
A competent boiler engineer would always be able to diagnose the fault on a simple boiler like yours.
Even if he did not know what the F6 fault indicated then he would have measured the resistance of the sensors and done basic checks on the boiler components. He would NOT just stare blankly and give up.
I do hope you did not pay those nupties. You need the fault diagnosed and fixed, they did not do that.