Some enquiries about radio packet systems came by today.  For a radio
packet network system, using PIC processors, see
http://www.sni.net/cedardell.  It is slightly out of date:

1. The two systems described on this page have now been merged into one
product
2. The systems use 10 channel digital tuned radio
3. Unit cost is low, helped by volume price of 16C57 and a shoe horn to fit
the code in.