I turned my central heating on today for the first time this year. The bathroom radiator (the highest one in the house) feels like it has more air than water in it, but opening up the valve to bleed it doesn't seem to have any effect. I had the combi boiler serviced during the summer and noticed, after the engineer had left, that the pressure had gone down from 0.6 bar to 0.1 bar. Does the pressure need increasing before it will be able to force the air out of the top radiator? How do I do this??