Thanks everyone for all the ideas and feedback. I *think* I have a solution that will work for me.
I'll change the default mutt temporary directory to something that I can mount easily on the laptop using sshfs, as long as this has a unique name that can be the same on the desktop server and on the laptop then files viewed using mutt should be in the 'right place' on both systems. I just need to write a wrapper to mount and unmount the directory when mutt is run on the laptop (or I could just mount it whenever I connect I suppose). This requires minimal reconfiguration - no IMAP server, no changes to my mailbox format - and fixes my current niggles (I think/hope!). Again, thank you everyone for all your help. -- Chris Green
