I don't have PV, but my sister in law has.
My understanding is that the PV measures the amount of energy generated which pays the FIT.
After this measurement it is a case of use it or loose it!
The electricity company can't measure whether this energy generated is used by the property after the Fit meter (emersion heater for example), or goes back into the grid.
My sister in laws system does just this, heats up hot water, for free, and it does not effect her FIT.
It makes sense if you think about it.
The FIT meter has to be straight after the solar panels.
If you then use it after the meter, you aren't pulling any energy from the grid, so you aren't charged by your normal electricity meter.