Had a bit of time getting this to work so sharing my final blueprint in case it’s useful to anyone else:
Also to get this working I had to use Vesternet’s own herdsman converters for Z2M as the bundled ones are out of date. There’s info here Vesternet Zigbee Wall Controller (8 Button) - Technical Support - Home, I used this one: zigbee2mqtt-herdsman-converters/VES-ZB-WAL-012_as_button_controller.js at main · vesternet/zigbee2mqtt-herdsman-converters · GitHub
You have to create a folder in your zigbee2mqtt data folder called external_converters and put that new file in there. On reboot of Z2M it should be using the new converter.