Stix...With your report of slight positive wing incidence it would appear that you have ruled out an incidence issue as the cause of the necessity for gross up elevator trim. That doesn't leave many plausible explanations beyond a good Aussie sense of humor.... but I don't really think you are jerking our legs.

If it wasn't an ARF, I'd be wondering if asymmetrical wing ribs were inverted.

It really doesn't leave much...

You said you are using flapperons. Are the ailerons trimmed to conform to the wing foil when flapperons are off and have you been flying with them off?

Are the stab bolts firm so that the stab is not moving up or down on the mounting bolts?

Are the wing bolts securing the wing to the saddle?