Thanks for the kind welcome, and for the offer of assistance, appreciated.
I've done a bit more digging and there's some oddities. I've dug into my use of Mackup and it appears that even though my Alfred backup files were in the Mackup directory, they weren't placed there by Mackup. I probably put them there myself. So I've turned on syncing in Alfred, and set it to back up to the root of my Dropbox folder:
$ ls Dropbox | grep Alfred
Alfred.alfredpreferences
Expecting that this should resolve the issue I then went to install Alfred 4.
Double clicked Alfred 4.app
Entered my Powerpack license information, clicked Activate Powerpack
Click Migrate Data…
Found preferences now shows as ~/Dropbox/Alfred.alfredpreferences
Found Local Data now shows as ~/Library/Application Support/Alfred 2
A warning is displayed at the bottom:
Due to the significant differences between Alfred 2 & 4, a few settings won't be migrated.
I backed up the settings just in case.
I get a few errors of the following types:
“Alfred.alfredpreferences” couldn’t be moved to “Alfred” because either the former doesn't exist, or the folder containing the latter doesn't exist.
“actions.alfdb” couldn’t be moved to “Databases” because either the former doesn't exist, or the folder containing the latter doesn't exist
I click macOS permissions
I give access to Accessibility, Contacts, but Full Disk Access is greyed out
I click Restart Alfred and receive error (see attached image)
Go to step #5
I'm not sure I mentioned this previously but I'm currently on OS 10.12.6. Does Alfred 4 require a newer OS?