LaurenceR:
The compiler only recognises it is a string if I use a Capital S?After some googling, I found there is a possible max of 82 characters (80 visible characters plus terminators).
char NMEA1[83];
char NMEA2[83];
82 plus the terminating NULL ( '\0' )
Unfortunately for you the Adafruit library uses the capital S String objects, but they probably didn't anticipate all the other stuff you have thrown in.