Support have contacted me, it's an MCX connector!
The one on the LoRa module is female, so the plug you buy needs to be an MCX Male connector.
I've bought these converters for SMA plugs:
https://www.amazon.co.uk/gp/product/B012THC0FO
The MCX connector is exactly the same one as on most of the SDR USB sticks... https://www.amazon.co.uk/TenYua-Digital-Receiver-RTL2832U-R820T2
So if you already have a USB software defined radio hanging around, the rubbish little twig antenna can be cut off the cable, and it reused as a J antenna cable!