Think the other rads need to be turned down. Turn hottest down a turn, then next hottest and so on. When you get close, eg your cold rad's warming up, adjust 1/2 to 1/4. This is an oversimplification of balancing, but it'll get you in the right direction.
Teeny update. Checked connection at PCB and at store and it worked for a short time, so new thermistor ordered. However, where it sits seems full of muck, which I can probably clean with a cotton bud or similar. Is this normal? And I take it the circlip just holds the wire and nothing else!