Thank you all for your recent replies. We have now had a new pump fitted, the old one was scaled up and obviously needed replacing. However, it would seem we now have a huge amount of air in the system although all of the radiators are hot and now working OK. The bubbling noise coming from the boiler and radiators is so loud it is keeping us awake at night - the plumber assures me this will disappear after a while (it was installed 4 days ago). However, this morning we did not have any hot water, the noise seems to have got louder and water was pouring from our overflow pipe. The plumber is working away this weekend and not contactable, so any suggestions in the meantime would be much appreciated!!