Thanks to all the extra replies.  For the benefit of those who don't know 
the full details cron dies because dump incorrectly took the parent 
process with it if it died.  Thus no tape in a dat drive for a dump 
started by cron would result in cron itself getting whacked.
The patch number is OSFV305-013-2 and it was sent to us by DEC support 
within a couple of days.  We installed it successfully and will not 
(hopefully) suffer the same problem again.
REgards from the Isle of Man
Stuart McKenzie
Received on Mon Jan 08 1996 - 16:01:32 NZDT