Petrol is about £1.10/L atm, or about £1.21 for the high octane which I use. Stops the engine pinking as badly, and it definitely runs better, I get about 50 miles more out of half a tank. I have known it to be nearer £1.40/L somewhere before.
Diesel keeps going a few pence lower than petrol too, so since Diesel goes further anyway, it makes it a bit more pointless in having a petrol car as it has a smaller range, and the fuel costs more.
But ah well. Que sera sera.