Thanks to the scrutineers at Callander this weekend ! Alan spotted that the nut on the base of my steering drop arm was loose, despite being held with lock-tabs. Not something I've have been able to spot because you need someone to shuggle the steering.
Anyway, a big socket and the 3ft tommy bar put almost an extra turn on the nut, so it was more than a bit loose - a replacement steering box was fitted during the rebuild, so I'm guessing the beasty was not torqued up properly then or has worked loose since.
A result anyway, Its safer and the steering has a bit less slop.
Thanks again Alan !