... for clarifying the Disk Usage display. Previously it showed a "Current" figure and I wasted a lot of time thinking it was current - it wasn't. Now it shows it as "Last reading" and says "Note: These values are only updated once a day". Much better!
And though it seems there's no way to get the Current figure from the panel, in the case of "Disk Usage by User" one can work it out from the output of the shell command "quota". If anyone knows a solution for "Disk Usage by Mailbox" I'd like to hear, thanks.