Hi Tredegar - SOLVED
I made my /etc/nertwork/interfaces file look similar to yours
I entered the "iwconfig wlan0 key restricted s:xxxxxxxxxxxx" and also set mode to "managed" along with the "ap_scan=1" option in my /etc/wpa_supplicant/wpa_supplicant.conf file (extract below)
I launch the wpa_supplicant process via the /etc/network/interfaces file (as a quick and dirty startup) so it's activated with "ifup wlan0" accordingly.
Amazingly, DHCP worked too (which staggered me) !
My prior downfall was presuming that the key statement was only required to be declared in the wpa_supplicant.conf file - Look like that's not the case.
All I need to do now is work out the correct method for clean startup and shutdown of the wpa_supplicant process - I'll look on the forums and documentation later today.
Thanks very much indeed for your help - muxch appreciated - now off to read up on clean startup/shutdown on process.
Sample code used below :-
Code : From /etc/network/interfaces :-
iface irda0 inet dhcp
pre-up ifconfig irda0 up
iface wlan0 inet dhcp
pre-up ifconfig wlan0 up
pre-up sh /etc/wpa_supplicant/start.sh > /dev/null & # My script to start wpa_supplicant
pre-up iwconfig wlan0 mode managed
pre-up iwconfig wlan0 channel 6
pre-up iwconfig wlan0 essid "MyEssid"
pre-up iwconfig wlan0 key restricted s:xxxxxxxxxxxx # xxxx= My key (WPA-PSK)
pre-up iwconfig wlan0 ap 00:0F:xx:xx:xx:xx # My AP (gateway/router)
pre-up iwpriv wlan0 network_type g
iface wmaster0 inet dhcp
pre-up ifconfig wmaster0 up
Code : From /etc/wpa_supplicant/wpa_supplicant.conf