Ah ha! I've got it!
KPPP lists the devices /dev/modem, /dev/ttyS0-3 but these don't all exist on
my machine. What I do have, however, is /dev/ttyS[0,1,4].
So I tried # ln -s /dev/ttys[0,1,4] /dev/modem and, finally, ttyS4 worked!
(Without any drivers or anything; it definitely is a hardware modem!)
Cheers for suggestions,
Richard