I could not for the life of me figure out this multistage process.
After a lot of trouble (even though I am a professional programmer!)
I finally got it working.
I will describe the steps as best possible,
having taken my steps from others' postings.
I am sure there is an easier way to do this!
Given filename: 01 SongName.mp3
Create title: SongName
Select Actions from the Convert menu.
Click the Star button to create a new Action Group.
I named mine: Strip leading numbers
Click the Star button again to create a new Action.
Select Action Type: Format value
Field is: TITLE
Format string is: $regexp(%_filename%,(\d*)\s?(.+),$2)
Click Ok to save.
When you are returned to the Action groups dialog,
make sure your new Action Group (Strip leading numbers) is checked.
Select the items you wish to have titled from their filenames in the main window,
then press Alt-5, then click Ok.
I think the Help could be very much improved for this excellent program.
The regular expression support is great and very powerful,
but is useless unless one can understand how to apply it.