How can I fix my Potterton Puma 80?

First of all we had two radiators that were not working, we bleed these two and they still did not work. We then bleed the whole system thinking this would cure the problem. When the system was refilled the radiators were working accept for the two that were not working in the first place. Then boiler then start hissing but when we added more pressure the hissing stopped. Then boiler then was running at 2 instead of 1.5. Now the Low pressure light is on permanent so the boiler is not firing up so we have no hot water either now, Any Ideas?