Just out of interest, I'm supposing you're running on Tesco's 99 RON stuff by default. Have you tried a lesser RON superunleaded alternative by any chance? Or Shell's V-Power?
I ask because my Saab has a love / hate relationship with Tesco 99 RON. Loves the extra octane from a performance perspective, but isn't as economical. I put this down to the bioethanol stuff that's used to increase the RON rating as having a lower calorific value than proper petrol.
Not sure if the 3p extra for Excellium is worth it over Tesco just yet...