Well, finally got the shaft back from Drive Shaft Solutions.
They determined that is straight but were unable to balance it.
So fitted it back on the bike with the FD and ran the bike in each gear and most of the vibration has gone. Even less noise when applying a bit of rear brake.
Conclusion is, the off road tyre has been the source of the vibration, not being very well balanced on the rim.New tyres on order.
The noise that I perceived as unsettling
Is deemed by a few others, "normal drive train noise". (Will check it against a similar model asap).
Feelings of stupidity but relief are washing over me as I write. Comments of guns being jumped and blind panic have been muted...
Will let you know how I get on with the audible comparison shortly.