I gave up. I tried everything. I tried getting a Grundfos balancing pump. It was better, but not great. I spent days and weeks. It was never good enough.
I re-piped the entire system with a manifold. Works like a dream.
On close inspection the original pipework was a total disaster. A large quantity of long runs in 15mm copper, 15mm plastic and additional radiators t-ed of 15mm.
Now I can see and measure that every radiator is getting the same: 4 l/min.