Well, I'm delighted to say that in 1 working day and 2:15 flying hours, the problem has been solved. In the process, I've certainly learned something.
We started by re-visiting the wing dropping stall. It was discovered that the wing drop was greater with more nose-down pitch trim. Looking back, in previous tests, if keeping height and power the same, inevitably the aircraft was trimmed further forward to maintain level flight at a lighter weight. I confess that this wasn't an expected effect, it was spotted entirely by accident.
On the ground we examined the pitch trimmer, which was through a single tab on the port elevator. With nose-down trim, the trimmer was of-course more trailing edge up. This initially made no sense, since one would imagine that a trailing edge-up tab on the port side would cause the aircraft to roll to port at the stall, not the starboard we were getting.
Then we looked at the elevator mechanism, and took some measurements. The port and starboard elevators were each linked through a pushrod, arm and pivot pin to a single pushrod in the fuselage. Through bending in the pushrod and a small amount of freeplay at the pivot pin, there was a remarkable (and previously unnoticed) 7° of available freeplay between the two elevators.
We theorised that the trim tab on the port side could push against the freeplay at the stall and create up to 7° of differential elevator. This was likely to be more marked at higher power, since the tail is right in the propwash.
Some rapid redesign ensued, and several washers inserted in the elevator lever arm joint, reducing the freeplay between elevators to 2°.
We flew it again, and although the wing drop was still there, so long as stall recovery was rapid, it was down to about 15° - which is within certification limits.
So, I end my day with the problem solved, I am a little wiser, and we might actually get the beastie certified on schedule. The company is inevitably very happy, since two extra washers is an awful lot cheaper than a wing fence or vortex generators.
Many thanks everybody for your help.
G