Thanks for such a fast reply.
The repair was to a pressure outlet pipe connecting to the Mid valve, which had bent was leaking, the plumber replaced and soldered the pipe, it doesnt leak anymore. We had to drain both the hot water and the CH system before making repair, which we did from the lowest point in the house.
After we switched the water back on I was given instruction to bleed the radiators, which I did, some radiators had high pressure pretty quickly (within a few moments of twisting the bleed key) others took longer, but water was coming out of all them with reasonable pressure eventually.
After which me and my neighbour who came over to help diagnose the issue ran the heating system set to the highest temperature for an hour, all the upstairs radiators were very warm, the downstairs rads were very cold. Then we checked the downstairs radiators by bleeding them while the system was running and the water in them was cold.
We tried draining the system again, and repeated the process above to see if there was air in the system. The problem still occurs.
An interesting side affect is if I set the water to auto and the radiators to off, the radiator nearest the boiler still comes on. And the hot water pump seems to come on for hours, infact it's not turned itself off yet today without me turning the system off.