Managed to syphon off the petrol by removing the plate in the boot and unscrewing the big plastic sealing ring. This was a little problematic in that it was difficult to get enough grip on the ring which was very tight. Eventually I ended up using a strip of metal against the ridges and carefully tapping with a hammer until it eventually moved.
This was just the start of my problems. Inspecting underneath I saw that there are a number of approx 8mm pipes connecting different parts of the tank. Unfortunately, when the LPG was installed the gas pipework and electrics was run in such a way that a run of the 8mm pipe I describe above is trapped by the LPG pipework. This makes removal of the tank impossible unless it is possible to disconnect either the LPG pipework or preferably the union of the petrol pipework. Here is a picture of the kind of union I need to undo. The actual one is at the front of the tank where I am struggling to get a decent picture:

I will need to remove the pipe at the union of the petrol tank.
Alternatively, I need to undo the LPG pipework at the tank so that I can bend it to allow removal:

It is the right hand copper pipe that will need to be disconnected. How do I go about turning off the gas so that this can be undone. Do I simply screw up the tap above it? System is a STAG.