Well!!!!!!!!!!!!, brakes still did not work and still got a clunk
Found that the pivot bolt for the operating lever goes through the whole assembly and the lever was binding and therefore the mechanism was not returning to the "no load" position. Unfortunately you cannot see this as it is inside the drawbar. This caused me to incorrectly adjust the brake cables. I caused this myself by going round the trailer and check tightening all nuts and bolts. Seemed a good idea ! However, it is bad design to not use a shoulder bolt in such and application and use a loose nylock nut.
PS: had to dispatch a 3 foot rattler today in the garage in the middle of this project. Many thanks to Jake the lab. for finding it and barking his head off.