Idle should be 500 rpm, when you can achieve that your vibration should have been discovered and where the problem is.

Have you checked to see that there is no part of the exhaust system touching the chassis ? The faintest touch will cause a buzzing sound , and movement of the exhaust may have resulted from replacement of the mountings

Yes noticeable at 500 rpm very noticeable from about 1300 upwards. Exhaust is not touching anywhere. Cheers PB

Might someone have forgotten some roller bearings in the new u-joints?
U-joint saddle play or deformation from the drive shaft being out of alignment as mentioned?

Nice thought Bud but vibration there when drive not connected.

Have been watching this saga from inception so disappointing that the vibration has not been found; so will add my suggestions!!!
With the engine running and the vibration occurring place your hand on the engine itself and move it from place to place to try and feel from whence it is coming (obviously be VERY careful!!!!). Also use your genuine SU tuning tool for balancing the carbies (piece of tubing!!!) to listen for the source. Put the car on a hoist as well and check underneath but always with the engine running. Do not forget that the engine is mounted at four points so check the front mounts as well, even taking the weight of the engine off them by jacking up the engine under the sump; no need to disconnect anything as you will be only jacking it up by mere mms.
The car has been out of your care and sight with someone else working on it; could even be something as simple as a spark plug lead knocked off and replaced out of order. Look outside the box!!!!!! as you might be surprised at what you find!!!!
Good luck!!!
Yours Kent

Hi , just a thought, what would be the symptoms of the drive train being put under torsional stress due to the mounts giving way and twisting , my thought is that if it twisted enough it may have moved the clutch friction plate out of line, result... vibration. only an idea, feel free to shoot it down in flames i'm no expert, but tell me why and I can learn something too. please tell us what you find and how you fixed it. good luck. JC

Is a fan blade cracked or bent, or a partly shredded vee belt?

That's a good suggestion, Dameedna.
If the original fan is present, It was balanced in production by the fitting of a rivet close to the end of a lightweight blade. If there is an hole but no rivet at the end of a blade, that will cause vibration.
Any bend in any of the fan blades, or the use of differing nuts and bolts to fix it to the tube or to fix that to the pulley and damper, will destroy proper balance.

Thank you one and all for all the great thoughts. After exploring all options with no success it has now been decided to do a full engine rebuild. A bit of a shock at first but it had been something on my mind for sometime in the future..... just advanced a bit now. If the vibration is still there after that I will great fully except any old razor blades you may have. Cheers PB