Realised there was some sort of blockage in the CH system after draining it down, as the system would not refill via the header tank, read somewhere on a plumbers /DIY site that the likely cause could be a pipe blockage somewhere between the CH header tank and pump, and it was likely to be near a bend in the cold feed pipe as it joins the CH hot water part of the circuit - it was, and so chopped out the offending pipe section and was able to unblock it and refit /replace using a couple of compression connectors inclusive of an isolation valve located above the repair so that it was easier for me to maintain/repair anything else should the need arise at some later point ie replace ball cock valve and/or connector without the need to drain the system down to work above the repair location.
Did take some photos of the rough job that I did, but its still holding up ok so far