This took about half an hour or so. The instructions at http://elinux.org/RPI-Wireless-Hotspot are pretty complete. The only slight issue I had was getting the Edimax Nano USB EW-7811Un I have to go in to access point mode.

There is a solution to this provided by Dave Conroy at http://www.daveconroy.com/turn-your-raspberry-pi-into-a-wifi-hotspot-with-edimax-nano-usb-ew-7811un-rtl8188cus-chipset/ - just follow the bit to do with installing the hostapd package itself. Don't follow any of the configuration stuff - unless of course you just want a wireless bridge, not a full-on access point.

My only added suggestion is that my DHCP server was failing to start because the wireless interface wasn't yet up and running. This is the error message from the syslog file:

I'm no expert in this area, but I fixed it by moving the service start command to the post-up part of the /etc/network/interfaces file, which now reads like this (the extra bit in yellow):

I also disabled starting udhcpd on boot with: