I have just moved house and the boiler - same as yours - was experiencing exactly the same problem, and when the heating was on the radiators were extremely hot. But the hot water could not be supplied when only "hot water" demanded. It turned out that at some stage in the past the "diverter valve", that determines whether to supply hot water or heating system, had at some stage in the past been installed upside down and thus heating demand was being interpretted as hot water demand and feeding the radiators with extremely hot water with full blast of gas - totally wrong. Boiler engineer reinstalled existing "diverter" valve in correct orientation (draining boiler and refilling - work took no more than one hour inc tgesting) and all was well. Plus ability to control hot water only if desired, without having heating on. This advice may be too late for you or may not relate to your "fix" but it worked for me and symptoms are the same. Best of luck.
Chris Joice
December 2010
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