The flow and return service valves are open when the line marking has the same direction as the pipe.
They were open, but I have success anyway - I switched boiler off, and then turned off all the TRV's on all but the bathroom rad, plus one other. I was then amazed to feel heat drawing up the pipe toward the one other rad I had on. I waited until that rad was completely hot before turning another TRV on another rad on, and one by one they are all on.
Not sure what relevence doing what I did had - but I have success anyway - Cant help feel I still have air in there somewhere tho.
Thanks to everyone who has had an input in this long journey - but its looking good so far.