Yeah, good shout… 
The device is a Heltec WiFi LoRa 32 v2 with an MFRC522 RFID reader attached to it (hence the challenges around hardware - the support for MFRC522 in ESPIDF is shocking, but the support for TTN v3 in the Arduino Framework using SPI isn’t great either as I’ve been finding out! 
)
The code has been taken from this post and amended slightly, but only to move it into a dedicated python function as opposed to a straightforward script.
I’m starting to think it’s something to do with the version_ids in the JSON, so I’m going to play around with that this evening and hopefully I can find out what’s causing this!