Final Post. Problem found!!
After the new copper ADF sense antenna did the same thing as the old steel wire it rules out the antenna itself.
Given that the problem occurred with the Coax disconnected from the antenna itself it seemed to leave only the coax attachment point as the cause of the issue.
I removed the ceramic post that the antenna terminated onto and found that the coax was supposed to join onto the bottom with a nut and washer, the nut and washer were sitting loose in the bottom of the unit. The connection to the coax was suspended just touching the bottom of the antenna mast, but would have vibrated in time with either engine RPMs or antenna movement to create the issue.
The worst vibration was caused by the engine harmonic and hence the RPM dependant part. The directionalility was indeed due to the directional nature of the sense antenna being aligned with the longitudinal axis of the aircraft.
Interestingly it was just a plain nut and washer, no locking mechanism to prevent it from vibrating loose over time.