I've got this working, it does exactly what i was looking for. It will join my home Wifi when i'm at home, and if my home wifi is unavailable (eg: i'm out somewhere) it'll configure a static IP address on the RPi wlan0 interface, fire up a hotspot and i can connect to that from my computer/tablet/phone and use the Rpi as a standalone device.
Here's the link i ended up following.
It's a little bit fiddly in that it seems to take a while to bring up the access point after reboot. I was debugging the autohotspot script and all i can see is that it fails while provisioning the interface due to device busy, but then after ~2 minutes it works (i run the pi headless so am looking at the logs after the fact.
SSid's in range: ++ iw dev wlan0 scan ap-force ++ egrep '^BSS|SSID:' command failed: Device or resource busy (-16)
The next thing i'm going to mess around with is if i can get it to join the "personal hotspot" from my phone/tablet and let me connect to it and have internet access from the RPi while i'm not at home.