So it's been a week, and about 400 km later, and I filled the car up to find that the reading is still a little bit out, but waaaaaaaaay better.
My commute to work and back (mix of suburban main roads and back roads) - after going over tyre pressures and such as suggested to remove anything that could be hurting my economy - cost me 10.0L/100km. Computer was saying 9.7 L/100km - so only a small difference there.
Then immediately went onto the freeway, about 130km non-stop at about 110km/h. I reset the trip computer at the start and filled up again straight away at the destination, and got 8.0L/100km - computer was saying 7.7 L/100km.
I'll keep tracking it and if it's continuously optimistic by that same 0.3 L/100km, I'll make another adjustment, but it seems a lot better.