How can I use Terminal to monitor a network interfaces total traffic in Mb I would like to use the terminal to monitor the network traffic on a given interface, with a view to providing alerts when certain cumulative totals have been exceeded.

The script needs to gain statistics from the point in which it is started, and there is no requirement to store values for future reference, or retain values over reboots etc.  I would like the output to be human readable, so values in Gb or Mb, not packets or bytes.

The intention is to be able to include this script in an Automator application that will run in the background, and open a dialogue box at set thresholds to alert me to when I nearing a download limit, allowing to choose to disconnect the modem etc.

I do not wish to download any tools or applications, and just wish to use the tools a standard installation provides for the terminal.  Getting data out of an application via applescript would be acceptable, but only if that application is also a standard tool, like activity monitor.
Sounds like nettop is what you're looking for.  Start it with the help parameter to see the options.

nettop -nc -m route seems to be the best overall traffic gauge but will need additional scripting to parse the output for you