hi fastferret
as you have pointed out your bb heats your water from the gas supply
you immersion heats the water with electricity which means if your gas boiler breaks down you can switch on the electric immerion heat .
having to work on these backboilers nearly every day my advice is get...
as i work for bg i thought the way the chap was talking he was gas safe registered my mistake i only joined 2 days ago and havnt got round to reading rules yet.
how do you get reg for the combustion chamber then ???
most common cause of this prob with suprima is pcb
but might be gas valve or gas valve solenoid
but my money goes on pcb
you will need a gas fitter to change any of these components
i would have left ev at supplied press , yes you top up press with air from a car foot pump
listen to the pump it will cavitate if full of air and be quite noisey. the air in theory should go to the highest point in the system- keep us posted
i would advise 1 shower fed from combi and install an electric shower so if the boiler breaks down you still have some form of hot water to wash with :D simples
check for split in aps tubes the become brittle with age
another thing to check is the venturi in the flue it dosnt take much to drop the pressure going to the aps