Finally after everyone's help I have managed to drop the fuel tank. Now I cannot shift the locking ring. I have tried shifting it using the screwdriver/chisel method but this has broken off some of the lugs on the ring. I tried a piece of wood to lessen the damage but this hasn't worked either. It does feel like the ring was tightened ith the tool and a wrench as its just not moving. I broke a gator grip trying to move it so dont think an oil filter wrench will work as its 18" around the ring.

Grrrrr suggested a hair drier which I will try as there is nothing to lose but any other suggestions are welcome

Don't try to tap it round from one place go around it bit by bit. If you don't manage it see this. http://www.rimmerbros.co.uk/Item--i-NTC6872

haven't done p38, however tapping fuel rings all the way round prior to trying to undo can release any tightness where tank has flexed

Finally done it. I resorted to soaking it in Plusgas as I figured that it might get to lube the threads even thogh its plastic. Tapped it out with a piece of wood. Cheers everyone.