Thanks, ballscrewbob. I've got a copy of the uBlox AT command reference and that's what prompted me to want 'lower-level' access to the U201 so that I could, essentially, build on Sandeep's excellent work on the MAKERGSM library and submitting a 'MAKERU201' library, for sending/working with other AT commands.
uBlox make a diagnostic utility (U-Center Windows) that needs a direct line of communication to the chip, i.e., through UART.
Overnight, I continued my research and stumbled across this page about SAMD. It suggests that pins 13 (RX) and 14 (RX) are configured as the onboard UART. I think that's the one I need to connect to in order to view the raw stream/AT command interface of the SARAU201.
So, now all I need is a 3.3v UART FTDI cable (I only have the 5v version to hand), and then I think that'll give me access to the chip directly so I can begin writing my own library and debugging etc.
If anyone thinks it'd be useful to share what I've done afterwards, let me know...
Cheers