I have a 120 litre LRA (I think) rear tank (cut a hole in your rear floor while the tank is out to access the fuel pump), Dolium tank in the right rear and 85 litre LRA tank from my County under the drivers seat. Fills by removing the seat base. I used it as an excuse to move my electrics up under the centre console. Had to drill the spot welds to remove the panels under the seat. Only reason I mention this is you may find a second hand early type sill tank and it will fit with a bit of work to remove the panelling.

Drilling the first hole is the most difficult.....................once you have one hole, drilling heaps more in your defender becomes much easier.

Installation is really not very difficult and you could pay for a third tank by fitting them yourself and with help to get them in position.

Rick