The following method produces a display only portal total. If you need to use this total in other calculations, a different method will be needed:
In layout mode, make a copy of your portal and reduce the number of protal rows in this copy to 1. (By copying your portal, you get one the same width and with exactly the same filter expression.)
Put a summary field defined in the portal's table to total the portal field from which you want this portal in the single row of this portal.
Excellent - thanks Phil