Yeah. Flex fuel is know to give drivers less miles per gallon.
In general Fordís Triton V-10 engines are known to consume a lot of fuel even in the pickup trucks they are in. I would suggest possibly replacing the spark plugs, clean the MAF sensor with MAF cleaner, check air filter, check for any oxygen sensor codes or just replace those to.
Perhaps you can get in touch with a local car tuner/dyno tuner who knows Ford motors and they can check your fuel trims and such.
I know here in the states. Some car tuners will do remote tuning and trouble shooting with customers and their vehicles. But it is a bit more cumbersome that way.
Perhaps contacting 5 Star Tuning. They have been around for quite some time and do custom tunes and such. Maybe they can help you or point you in another direction. Iím thinking of using them for my F350 truck myself for a bit more power and transmission tuning.