Emma,
Switching 'Grader Report admin settings' to 'on' achieved the desired affect.
It is now possible to 'edit calculations' of new grade items.
It remains a mystery though as to why existing grade items with calculation could be edited with this 'Grader Report admin settings' set to 'off'.
The moodle default setting is 'on'.
I have tested this on two different 3.3.2 instals on two seperate servers.
One of the 3.3.2's was a clean instal, and on instal this setting is 'off'.
The second 3.3.2 was an existing earlier version moodle with courses in that had, grade items with calculations, and that 'Grader Report admin settings' would have been set to 'on' before it was upgraded. On moodle upgrade the 'Grader Report admin settings' seem to switch to 'off'.
Useful info for anyone following this thread. (and maybe for moodle HQ as these findings seem a bit unexpected)
Thanks for the nudge in the right direction Emma.
Cheers
Peter
PS. Just switched 'Grader Report admin settings' to 'off' again. and still the 'edit calculation' can be seen and edited. Odd?