SUMMARY: /usr slowly fills

Thanks to Johan Brusche, Peter Stern, Tim Cutts, Terry Horsnell, 
Howard Arnold, BL Venkatesh, Werner Rost, Marc Schlensog.

The quickest of the various approaches suggested:

touch marker
find /usr -name "*" -newer marker

gives a list which can be monitored to see what is growing.  It 
points to a culprit directory:

/usr/var/spool/mqueue

which has been filling up in fits and starts.  I'll post a further 
summary once I've worked out what is doing this.

Cheers
Derek