hi mate im a mazda tech and the mazda 3 is the same car with the same running gear .the focus and the mazda 3 has a plastic tank which is mounted above the rear suspension bed for the dpf (diesel particulite filter ) it a way of lowering exhaust emmisions
the fluid in the tank will be emply the fluid from mazda is £100 ish plus labour we charge 1 hour .
you might need it for mot as the exhaust emmisions will be higher .
you you D I Y it be very carefull as the fluid is highly toxic