When I hit 60mph the steering wheel shakes, not just a little, it moves around 3-6cm in both directions. if I hold the steering wheel very tightly the vibration can still be felt further down the steering colomn. Yet if I loosen my grip the steering wheel goes crazy but the wheels still drive a smooth path???
Once I hit 67-70 the steering becomes completely smooth.

I phoned my local tyre supplier, he was convinced it had to be the tyre balance... so I had them balanced, still the same.

On the same sort of subject, I had the car at a local "service" centre and he was turning the steering wheel while the car was stationary. The power steering unit sounded like it was really struggling... although to me all cars' power steering struggles if stationary - I think the guy was trying to get more work out of me to be honest??? but if not that could be related.

try swapping the rear wheels for the front ones. if its the same youll know its not the wheels/tyres.

other causes could be the steering shaft uj. or maybe the swivel joints.
poss any of the steering joints.

Thimk you will find its the bearing pre-load on the swivels. This should be set to (from memory ) 14 lbs with the wiper seals in place and the steering arms disconected.

Yep you were right, opened up the upper and lower swivels pins and found the bearings had slightly worn so that there was too much clearance the swivels I had to remove a few of the Shims to get it to sit correctly.

Have ordered a new set of bearings!

fitted new bearings on one axle, one the other axle i just had to remove a 0.03 shim which closed the "wear and tear" gap and gave the correct amount of torque / resistance on the steering.