My central heating sysyem has started to make loud banging and knocking noises. This only starts about an hour or so after the heating has gone off. I recently had some plumbing work done which I think meant that the meant that the water to the radiaters was switched off. (you can prrobably gues I am no expert) When the work was complete the plumber forgot to switch it back on and now i can't seem to get hold of him. I switched the water on myself and noticed that there was a lot of water going to the overflow so i turned the water down. A week or two later I put the heating on and all was fine, but I was awoken an hour or so after going to bed by the pipes around the boiler knocking very loudly. No matter what I did it wouldn't stop, which was I bit frighening, but enventually it stopped. I tried it again a few days later with the same result. I would like to know if this is this dangerous and what I can do about it? Do I need to slow down the the water flow to the boiler? The boiler worked fine before the plumbing work and to be honest money is really tight and i was hoping this is something i could remedy myself. Help!