We had 2 radiators taken off the wall and put back as we had some work carried out. This did cause problems with the boiler and we had a plumber to replace all the washers in the boiler. We have noticed recently that the pressure is nearly zero, but also we can't get hot water unless the heating is on. Could this be linked? Is there any other reason why we can only get hot water when the heating is on? Any advice would be appreciated :-)
Is this a heating boiler or are you talking about a hot water tank? If it is a heating boiler then it should only run when you turn the heat on otherwise you would be heating your home mid summer. If the hot water tank only runs when the home heat is on you have a bigger problem. As far as pressure double check that all the valves are open.
As David has said its a pressure problem and you need to repressurise the boiler, Most likely on your boiler is a built in filling loop which is at the very bottom of the boiler and roughly left of centre,it should be a hole about 3/4 inch diameter into which you fit a plastic key(unless its been left there). The key is turned 90 degrees and a smaller valve close by also needs to be opened to allow the water into the boiler, I recommend 1 bar cold and you may need to bleed some of the radiators and then top up again.
Firstly you need to get the pressure up. Worcester bosch don't make this easy. It might be you have a white plastic key that needs to be put in underneath for you then to turn another piece and fill the heating to 1.5 bar. Yo may have a conventional filling loop with a flexible hose. Best check booklet but it should help if you top up the pressure
Your boiler water pressure should be between one and a half bar and two bar.
