Hah! Licked it at last. After replacing thrust arms, lower control arms, tyres, getting wheel run-out checked, alignment and wheel balance done twice and still having problems with the dreaded 80 -100 km/h shimmy, I finally got around to replacing the tailshaft centre bearing to solve a different vibration problem. Found the gearbox mount on one side completely separated, and the other one just about to go out to lunch. BMA provided new ones and I fitted them today, took the car for a drive and VOILA! no front end shimmy either. I suspect the broken mount allowed the whole engine and trans assembly to oscillate gently, which transmitted through to the front end via the engine mounts. As one of our fellow enthusiasts said "the process of finding solutions to each of the problems we will experience with our E34s is an adventure of unparallelled satisfaction".