> Let us hypothetically take the average MPG for the diesel as 35 and that for Petrol as 20.

That's a much bigger difference between petrol and diesel than I have ever heard about, e.g., I read this about a MB Diesel -- "The diesel E-Class's fuel economy rating is 26/35 mpg, for a combined 30 mpg, and compares with 19/26, for a combined 21 mpg, in the gasoline E350.