Apologies, my last post on this thread was rather off topic, was just thinking aloud
To redeem myself, I've just excel'd last months' fuel usage in petrol, and compared to my current diesel's average on same journey.
Petrol: 1408 miles @ 37mpg (avg) @ 1.38/litre = £236 in fuel (24 sept - 24 oct)
Diesel: 1408 miles @ 46mpg (avg) @ 1.42/litre = £195 in fuel
That's a clear £40 cheaper in that month, £480 per year extrapolating that out!
Even just working out a single month, that completely tallies up with Seven7's figures
In case it's helpful, excel sums are:
Miles / MPG * cost per gallon