Take the TRV head off and check the pin underneath is not stuck down. You can do this without even turning he water off, it's the very big nut on the head itself.
If you feel the pipe going from the three port valve to the cylinder and its getting hot then the three port vave is staying in the open position which means it is broken
Putting a new combi on a one pipe system in my opinion is a bad idea in the first place. When the new boiler was fitted was the system power flushed? It sounds like you have alot of air in the system from when it was drained down and that's what the banging is. Is the boiler firing on HW demand...
I've been looking for some ceramic tap valve replacements and can't find any that fit. The closest I've got is the ones they sell at bnq but the spindle is just too short. Can anyone help? The customer is prepared to buy new taps but surly there is an alternative? Any help on where I can buy...