Thanks, John, for that vital additional information. It sent me back to the camera manual which has this to say about using non-dedicated flashes:
"For use with non-dedicated electronic flashes: Set the mode selector to the MANUAL ADAPTER position and the Adapter at 1/30 sec. (Although the flash is synchronized between 1 and 1/30 sec., 1/30 sec. is most recommended to avoid camera vibration.)"Elsewhere on the Internet one finds the advice that the Manual Adapter must be set to 1/60 sec or slower; on my camera, both speed settings seem to fire up lightning symbol in the viewfinder.
I shall still have to experiment a little and let the resulting negs resolve the doubt. Thanks again for your input.