Currently Being ModeratedNov 22, 2012 4:21 PM (in response to Steve_R_Preston)
To partly answer my own question:
The mailboxes are not on my Mac!
Multiple messages in Trash disappear on emptying the Gmail Trash folder by logging directly into Gmail.
There does not appear to be a way to manage this from within Mail but I would be happy to be corrected.
Five minutes spent on this = massive improvement in productivity.