Thanks to Superuser I've just found rdiff-backup.
I've read the man pages and had a fiddle with the --include parameter but it did not do what I want, so my question is this - Is it possible to use rdiff-backup to back up a single file (which is located in a directory with a number of other files I am not interested in backing up) ?
Rewritten
If you set the source as the absolute path to the single file you want to transfer, that will select that one file.
If you need to backup a single file, let's say "/home/user/important-file" to "/home/user/backup" do this...
rdiff-backup --include /home/user/important-file --exclude '**' /home/user /home/user/backup
