Pete,

 

wpa_supplicant_wlan0="-Dmadwifi"

From your previous message, your are using the "madwifi" driver. However, you are executing 

wpa_supplicant -Dwext -iwlan0 -c/etc/wpa_supplicant/wpa_supplicant.conf

Where now you specify the "wext" driver. Maybe that's the problem (?)