I took the clutch apart on the project, cleaned it and rebuilt it making sure there's new oil on the plates. I've come to set the free play and it's solid.
I took the sprocket cover off to cleck the slave mechanism and it all works, so I'm going to go into the clutch again

Any ideas what mistake I've made ???

Boris,is the pressure plate (cover plate) aligned ?
It will fit on numerous ways on the splines (5) but will ony release (lift) in one position.
Could a spring be jammed in the recess ?

Pressure plate sounds like it - I was thinking along those lines. There's a part that fits into it too I wonder of thats lined up.
Well only one way to find out I suppose

This is weird.
Checked the plate and it lined up. Undid the retaining bolts and gave the clutch lever a squeeze, it all seems to work. But do it all up agathey're as the bolts get closer to touching down so the amount of movement becomes less. When the bolts touch down the movement is zero.
I measured the springs and they seem the right length. The only thing I can think of is the springs are coil bound there seem to be 7 windings on mine and theyre red oxide coloured.

Do I tighten the bolts so they touch down or a number of turns out ? The (600) manual gives a torque setting so i assume theyre done up.

Bolts are always tight.

Coil bound, as you state.
P.O. has had clutch slip or replaced plates and then fitted incorrect springs.
This may have worked ok previously as bolts were not tightened correctly.
When you overhauled the clutch and tensioned bolts to spec, the problem arose.

I'm going to take it all apart again and reinstall it ... Can't do any harm.

That's too weird - opened up the casing an a mate walked by. I explain how its binding and do the bolts up one by one and he pulls the lever after each bolt goes down ... It works fine

Are there any wear grooves in the clutch basket? One or more of the plates might have snagged on a groove when you tightened the bolts up at first and pulling the clutch lever as each one was tightened up just freed it up

Maybe ... But it works now so I'm not going to argue with it