@IonC Hello, I was randomly browsing briskoda, and have seen this topic. I may help you, I had a similar issue, juddering/misfire at heavy acceleration and on higher rpms. Also fuel consumption increased. DTC popped up on VCDS, but none on the dash. All of them were misfire related, BUT seems to be caused by 2nd exhaust sensor (commonly referred as lambda probe). After exhaust sensor replacement problem went away, consumption went down-normalized. Used BOSCH 0 258 030 221 (my Octavia is 1,2 tsi CJZA), but check if it fits your 1,4 TSI as I don't know which code is used in Australia.
Worth mentioning that i've changed the spark plugs at the same time as a precaution since the car was obviously running rich due to faulty sensor.