I have already lost one tank on my lightweight to rust, and the other is following suit! As I would also like to fit some "comfy" seats I figured this would be the ideal oportunity to fit a civilian style tank with an external filler, instead of having to remove the seats to fill up.

Anyone out there done this? Any probs encountered along the way and ways round them would be helpful.

I have access to some "donor" vehicles from which I can probably obtain filler necks, caps etc.

Any info appreciated!

Not done it or seen it done. But I have seen a LW with an external filler-cap: the owner had modified his existing tank set-up.
Due to the unique layout of the LW I'm not sure if there's anywhere a civvy tank will fit. And besides, half the fun of owning a military Rover is watching people's reactions in the petrol stations
You could try asking here: http://forum.emlra.org

Gareth