If your installation is matched with diagram then you may have to look in to expansion area. There is a company called surrygas.co.uk try to speak to them. they are very small but best in heating business. masters of heating problems.
If you have this problem for last 13 years this could be a wiring problem however this not hard to check remove the top part of the mortarise valve and manually turn the valve to isolate the hot water system and try to see you get a better result
you have lots of dirt in the system this why you are experiencing both radiators and the hot water problems fix for this problem find some one good who can do the power flushing but before you do the power flushing remove the DHW heat exchanger and clean the primary channels and put it back this...