How do I fill the system with water?

what the fuc you going on about ?
read the question the pressure gauge is on zero that tells you it's a sealed system. so why would you hav a tank in the loft ?

hi-spec plumbing
August 2007

Is there a small tank in the loft ?? That's where the water for the heating will come from. There's normally a small ball valve in the tank. The hot water is water taken from the larger tank in the loft and piped through the boiler and heated then stored (unless you have a condensing boiler) in the hot water tank in your airing cupboard.

Mark A
August 2007