What colours are the wires in the plug?
My auto Disco used to do 25-30l/100, used to struggle to get 400kms on 100l tank! At least yours can do 250kmh...
Have you had a gas analyser on it? Would be a good start to figure if the mixture was rich. Check what fuel pressure it has in the rails and make sure it holds the pressure overnight. Would also pay to pull a few injectors and check the spray pattern, if any show poor pattern, get the whole lot serviced.
you need to put a stick shift in..