Stu, try turning the concentration knob to the right (clockwise), on mine this increases the concentration.
May be the other way around for you though, just a thought.
Interesting, so my US spec Z that I currently run on 98 octane is actually mapped for the US 91 RON rating? And its 91 in California and 93 octane elsewhere.
My papers from Nissan say 214kW (287HP/371NM. I thought the EU version was 276HP?