We lifted the body off with the tank and plumbing still in place (cut the fuel pipes at the engine end)
We managed to pivot the body up with a 4×4 trolley jack on a piece of rafter off-cut to get up to a decent height, then the two of us lifted it up so it rested on it’s side.
On it's side
The tank is held in with 4 large bolts (14mm) which came out easily enough. We unbolted a few more pipe brackets from the underside. Then we removed the fuel cap and cut the rubber membrane that the filler pipe goes through from the boot with a sharp Stanley knife.
Rubber membrane cut to feed pipes through
There is a little hook welded on the chassis that holds the tank in place at one corner so with a bit of jiggling the tank came free as one of us pulled it out and the other pushed the filler pipe through from the other side.
This is what the tank looks like. You can see the bolt holes at each corner (shiny silver rings) and the filler cap on the right hand side of the pic – this is the bit that needs pushed through the rubber membrane from the top of the car.
Fuel Tank removed
This is a little fuel canister thing that sits just in front of the tank bolted to the body. I believe that this thing is filled with fuel so that if you run out, there is some left in the system and you don’t get air in the fuel lines.
Fuel Canister
@ andy9eleven 
That little fuel canister is a fuel/vapor separator, not a spare fuel tank.
Thanks for that! Didn’t really know what it was, just going by what someone had told us. Guess we better keep it then on the Exocet.