Update: I had a plumber come in and all is sorted. The system just needed bleeding and the pressure topping up. I had been way too tentative with opeining the bleed valves as I expected any air to come out first. Done properly, with the valve(s) full open and water the air then water blowing into a 2 litre container for a period, the air lock was cleared but it took a lot of water/pressure out of the system. Topping it up was by an external mains feed and hose into an inlet at the bottom of the HW cylinder return. The plumber was great (can I mention his name here?) in that he explained every step. Thanks everyone for your help.
