I would like to thank the people that offered help & advice, I have fixed the problem & hope the solution may help someone in the future.
I went to a diy store to look at the workings of a new valve so I could see what the inside of my valve was doing, that increased my knowledge.
I checked all circuits again & couldnt find a problem, so I went back to the the starting point - low pressure.
I part drained the system via the lowest radiator (decided to take the valve of while I had the chance), then I refilled it SLOWLY, making sure to manually put water through the HW circuit & the CH circuit, until it reached pressure. I bled the radiators & some from the system pipe & left it over night.
Next day switched both circuits on independantly & both worked, but they were both noisy, I bled again & left another day. Now work normally.
I spent just 99p & gained a lot of insight into the workings thanks to the input of others (well not PLUMBERS1) on this DIY forum, I hope I can repay the favor on the various IT & Photographic forums I participate on.
DIY how to tutorial projects and guides - Did you know we have a DIY Projects section? Well, if no, then we certainly do! Within this area of our site have literally hundreds of how-to guides and tutorials that cover a huge range of home improvement tasks. Each page also comes with pictures and a video to make completing those jobs even easier!