Tried again tonight ... another 4Hrs
Put trailer on stands so all 4 wheels clear of the ground.
Applied handbrake .. can’t turn wheels in reverse
Added play into actuating rod until I could rotate in reverse, around 30mm of play. Way too much as rod sags, but did it as a test.
Hooked up car .... same problem, rear wheels locking up.
Spoke to a 2 trailer mechanics who said better to manufacture a clip to put in place to prevent compression of coupling., and thus preventing brakes coming on.
This was way reversing was dealt with prior to these auto reverse systems .... posh 8nstalls even had a solenoid operated clip so it could be applied at flickmof a switch.