Hello Mr. jez Smith,

A good descriptive article about
amateur radio packet radio can be seen
at the below set site. They have solved
the same problems about 15 years now
that you are going to get into.
No need to re-invent the wheel.

INTRODUCTION TO PACKET RADIO
By Larry Kenney, WB9LOZ
http://www.choisser.com/packet/

Good luck,

Regards,

Charles Bannister