Ahhh,
So to be clear (and to help any others who might stumble across this) -
To setup WIFI networking on the SD card when not using the installer
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Create a file called “preseed.cfg” at the root of the SD card
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Populate the file with the following information:
d-i target/storage string sd
d-i network/interface string wlan
d-i network/auto boolean true
d-i network/ssid string YOUR_SSID
d-i network/wlan_keytype string 1
d-i network/wlan_key string YOUR_PSK
Where YOUR_SSID and YOUR_PSK are the SSID and PSK for the network you wish to join.
Hope that is all correct? Again many thanks for the super fast replies - really appreciated 