Thanks Lorne.
Just to confuse things a bit more, I found this review on 'Optimax'
http://www.ciao.co.uk/Shell_Optimax_Petrol__Review_5329606 And here's a claim i'll follow up on
"Independent tests have shown the acceleration increase from using BP Ultimate Unleaded is roughly equivalent to upgrading from a 2.0 to 2.3 litre engine "
http://www.bp.com/sectiongenericarticle.do?categoryId=4005565&contentId=7009016 Although on a quick first glance at the 'independent test review', I see that the test was on only two cars (BMW M3s), and although the test was repeated, they don't say if the fuels were swapped over at any point...
