Another race-pace full day. The rep said that there didn't appear to be anything wrong (that code), and that often just turning it off and then on again will resolve it.
He directed me to the Roland site for a firmware update for the printer, which may have been the problem - a disconnect between the existing firmware and my up-to-date VersaWorks. Dana?
I did another 4' x 20' banner print today and it went ok, so......phew!