Simon:

[...] if I make any changes directly in the text file, do I have to do a "makemap" to update a db file?

I think so. Sendmail may try to do it itself if it notices: I can't remember if that was standard sendmail, or some FBSD tweak, though.

If so, can anyone help me out with the correct command please, [...]

Sorry, I normally have to crib from the man page for makemap.