Suggest that you need to install a pressure relief bypass valve. Sounds like you have only just enough water flow through the boiler when all rad valves open.
Hope this helps.
Rich
The central valve is for aesthetic reasons. You could use the end connections but you would have to plug the central connections, which may look untidy.
Have you looked at other manufacturers such a Bisque or Zehnder?
I think that the designer rads in B&&Q are made by Zehnder.
Hope this...
Could be TRV on wrong end as previously said. Some TRV's have an internal adjustment for flow or return operation. Could also be plastic pipe moving.
Rich
Reading the OP's post again I have spotted my own error. The primary flow and return connections just need to be on the 'outside' of the secondary flow and returns.
Rich
Don't think it matters much. I have seen commercial installations with connections only 200mm apart. The important thing is that the header is the correct diameter. The 'low' refers to low pressure loss which can be acheived by limiting the velocity of the water in the pipe to below 1.5m/s.
Rich
You say that you only have 55c at the flow connection to your lounge radiator. There lies your problem This should be more like 70 or 80c. What is the temperature of the flow pipe from the boiler? Can you turn it up?
You also have a TRV in the same room as the room stat. Turn this to max or...