Well it turns out that the HP un2420 cellular modem has an inbuilt GPS that can be turned on and off.. So a bit more google foo and I found all that was necessary is to echo a command into the third enumerated serial port like so;
#$ echo "\$GPS_START" > /dev/ttyUSB2
Then we can just point our favorite GPS monitor program at /dev/ttyUSB2 and we should see NMEA data. I used gpsmon to test mine. I was amazed to watch this working on my bench inside the shack.