Found the cause of the rattling, a duff steering UJ, soon replaced thanks to the excellent service of Beamends. The original steering shaft was fitted upside down, resulting in my ordering the wrong bit at first.
Whole job only took a couple of hours, luckily the column slid up & down the splines fairly easily, facilitating removal.
There is still some wheel wobble, but at least the terrible rattle has gone. So much for the garage who checked it twice and could find nothing wrong.
Final question: What is the correct way to adjust the steering box? I've tightened mine up trying to cure the rattle and have now released it a bit - with no apparent effect either time.
TonyB