How do i fix a dicky fuel gauge on my sons newly aquired focus 2lt ST?

My son has just bought a focus ST with only three months warranty on the engine and gearbox , the fuel gauge has gone dicky and on most of the cars i have owned the sender unit can be found under the boot carpet or the back seats,having breifly looked under both it cant be found, what do i have to do to fix it. Thanks in advance if you can help with this one.