Thanks!
Forgot to mention, my fill this morning was 46.78L ... that's the average fill when I wait until the fuel light comes on!
So with a 46.5L average fill I calculated that I could do over 860km (534 miles) with one tank if I was able to maintain my average FE of 5.4l/100km! I had to do the maths 5 times to be sure, I couldn't believe it and thought I messed up the calculations!!!