OK, swapped a few pulleys around and now got a 2:1 reduction from motor to spindle, and I think I can improve that with a further change to get 1:1 if I change the motor pulley a bigger one.

At 2:1 and '50Hz' on the VFD I reckon I am getting 700rpm (4 pole motor) at the spindle, but I need to take it to 52Hz to avoid a nasty resonance in the drive train, bench mounting or whatever...(think its the flat belt joint 'pinging' the countershaft mount into vibration, may need to put another piece of 19mm ply under the bench top in that area to increase the rigidity of the top, or maybe some lengths of 2" angle-iron)

Anyway, running at higher revs makes for a much better surface finish.

I tightened up the top and cross-slide gibs so they are fairly stiff but I'm still getting some movement somewhere which I think is contributing to the 'taper'. If I wind the cutting edge towards the chuck on the top-slide, taking off a moderately fine cut, if I reverse direction the tool digs in and cuts deeper, yet I can't detect any significant movement in the slide itself. Any ideas how I can validate this? One thing I was going to try was to put the DTI onto the bed and see if there's any movement in the saddle when the topslide is wound in for cutting, but I can't tighten the saddle gibs any more else I can't move it!