So, having fixed my previous issue of not being able to connect to my WLAN, I now have been trying to get a remote connection to the ABS via the VAP.
Here's what I did:
Backup the wpa_supplicant file
Remove my WLAN details from the wpa_supplicant file, to mimic having no WLAN available
Restart the Pi
The device restarts and eventually (it seems to take some time!) the astroberry SSID is visible, but I cannot get a connection to it at all - I just get "Activation of Network Connection Failed" on my Ubuntu 18.04 client.
ifconfig shows