Can you let me know the size of your system and the export readings please Neil?
If you had a 4kw system installed, even with optimum generation there is still no guarantee all units would be exported, your property would use what is being generated before exporting. If everything in the property was off at the mains then of course this would go back to the grid.
The Export menu will record what is being exported. I would recommend using today as a reference point and in 7 days you will see what's gone back in kwh, then you can cross reference this against your usage and then asses. I am confident you will be using more than you have exported.
If you want we can have a look at your account and readings etc? Pop me a message and we can see if we can get some more information on generation and usage, this might help.