Well, I seem to've fixed things simply by fitting the spring from the old clutch. I know springs can be variable. I'd looked at the new spring & as accurately as I could measure it seemed true, not so it seems. But I find it hard to believe the new spring could be so bad as to cause that much drag. Repeating, I could pull the handlebar lever in & the clutch simply didn't work.
I've put all the plates in to fit the pushrod, I'll sort a shorter one & try taking out the 2 plates.