Thanks for the info Amanda...would love to see some pics of the tanks.
I have also noted that from Gardiners site that a 500l flat tank is
Height 620mm
Width 920mm
Length 1000mm
Cargo bed length 1900mm or 74.8 inches
Now I have from landrover the width between the wheel boxes is 925mm or 36.4 inches.....so a 500l flat tank...should

fit between the wheel boxes whilst still allowing for 900mm cargo bed length....please do check and do not take my word as gospel

Looks like a 500liter tank it is then...

Dave.