Dak
S-M, just a few thoughts to go along with what ever has been said.Was it a left w/t that was replaced, as a friend says its usually a sign it may have been g/looped,,repaired/ not recorded etc.Have you done a full symmetry check ,ie w.tip- tail-nose-w.tip, as if its bent ,it should show-up. Are the engine thrust -lines correct iaw the manufacturers spec.? Are the flaps up correctly, not sagging on one side or drooping, same with any engine intake ducts/cooler outlets.
Are the clearances between control surfaces and wing/tailplane /fin structues correct, as if not this can lead to poor control etc.In level flight are the positions of the ailerons noticeably different, looking out of the windows? Does the angle of the aileron trim agree with cockpit indicator? Is there any lost-motion on the tabs due to wear, as in flight this will give a false trim position?
And now you probably need the test-pilots highly expensive piece of test instumentation; a piece of cord( parachute) ,about 10-12",knotted at both ends/sealed, and some speed tape.
Fix one end on the centre-line of the nose(aircraft), about 15-18" in front of the w/screen centre, using the speed-tape; at the end nearest the w/s, mark-off 5 and 10 deg. increments, left and right, but visible to the pilots/observer, up to 20-25 deg.
You now have a sideslip indicator, more accurate than the t&s, and I would suggest you look at progressive sideslips, left/right, to determine if the aircraft is normal. Forces and displacement should be progressive with increasing sideslip-angle, and also with increase in airspeed.Fixed power should be used at each speed, and speed maintained, by slight climb/descent.
Any serious non-linearity should be cause for concern and further investigation.
As you dont say where you are in SA, I have an ex-student in SAAF who may be able to advise.Go carefully !!