I finally managed to get the NTX2 and GPS working together thanks in no small part to NigeyS and fsphil from UKHAS.
One small problem still is that the RTTY signal from the NTX2 has a small “blip” in it that destroys the string. This must be an issue with my code somewhere as I can run some demo strings through the NTX2 using some simple code with no problems.
See the blip? Not sure what is causing it but I guess I better find out!
The blip was due to a drop in voltage. The GPS was drawing more current than the power source could supply. Plugging in a 1A 12V PSU fixed the issue.