[Evolution] How to delete single messages from the trash bin

[Bradley G Ward <barnoo bigpond com>]

This may be, and probably is, a basic question.  But how can I delete
single messages from the trash bin so that I can reduce the amount of
messages that are in there?

I've searched the Help option which tells me how to expunge an entire
folder but I can't find how to do it for individual messages.  I can
highlight the message but the option to mark the actual message is
greyed out.

Thanks for any advice here.