Thanks for your response. Yes I am competent to do the job. I have the installation instructions but they don't help. The issue is that although the old and new are the same model connecti g it to the existing wiring and controller leaves this grey cable over despite all old connections being in place and connected. On the old boiler a wire comes out of the heater element, loops back into it. On the new boiler the grey cable comes out of the heater element, does not loop back in, but comes out to the three black (1,2,3) and the green/yellow earth cable. In the instructions this grey cable should connect to the new controller. But since the existing controller is connected in exactly the same way as with the old boiler, I am puzzled as to what to do with the new grey cable. I wonder if I just insulate it and leave it UNCONNECTED would that be ok since the old controller is connected as initially. I am also wondering that if I do that, will the old controller be ineffective? Sorry for the long reply, but it is a real teaser.