I just followed the instructions given here for setting up a wireless hotspot to connect to my Pi with.
However, I think I must have done something wrong.
As I am using a headless Pi, and have no screen or keyboard or ethernet, this means I can't access it anymore. I also can't edit the ext4 portion of the SD card from Windows.
Is there any way reset the WiFi behaviour so that it tries to connect to my router again, and so give me access to my Pi?
Update: In the end I realised I had a SD card with USB adapter lying around. I installed a new OS on the spare SD, and fixed my other card from there. I think the question is still valid. Since you can set the wpa_supplicant.conf file, and enable ssh by putting files on the boot partition, I would have thought you might be able to do other things.