I have tidied up the board for my second biot node, mounting the battery and up-convertor onto the PCB.
Annoyingly I have run out of ATSAMR21B18-MZ210PA modules so can't build any more at the moment :(. They have been on back-order from my supplier for a few weeks. Not sure how long I will need to wait.
I could start looking at rolling my own board using a standalone SAMR21 chip rather than the prebuilt module but that involves a jump up in my fabrication and design skills and also means the issue of radio compliance/certification might need to be considered.