Hello all, I am no plumber but have always managed to at least top up the pressure on every boiler I have owned. Just moved into a new place and there is a British Gas Branded Worcester-Bosch CDi Combi. The pressure has dropped right down which tells me I need to top up the system. But all I have is a plastic key and a plastic black hole to put it in.. Can someone explain in a step by step guide what I am supposed to do? There's no instruction manual left here so I stuck.. I could have taken a chance and turned something but didn't want to play around in case I break something! Help appreciated, cheers. R.