Pressure on my Remeha Avanta Plus has dropped to 0 after street work on gas supply. In the manual it says I have to turn OFF the boiler at the mains before topping up, but when I open the taps nothing happens. Surely the boiler needs to be ON before the pressure can increase, or am I wrong? It just worries me that there is a big warning in the manual to switch off the boiler while topping up! Any help greatly appreciated...