Last week I noticed that the central heating pressure gauge on our gas boiler was low. I had recently bled some of the radiators and had not realized the need to repressurize the system after doing so (see here for a good explanation). However, although the boiler user instruction manual mentioned the need to repessurize, it did not give any details on how to do it. I even looked at the installation manual, but that was rather forbidding with all its warnings about requiring competent, qualified engineers. I was just about to phone for a plumber when it occurred to me to do a search for a solution on the web. To my delight I found a Youtube video which explained exactly what I needed to do: "Filling Boiler Worcester Greenstar combi". Thank you Abbey Mead Heating!