I doubt the newer car will net you 10 more MPG, the trip is short, the engine will have barley just got into its optimum by the time your almost at work.
Why diesel? 13 miles is still very much in petrol territory I'm surprised no one has mentioned it. Its only really 6250 work miles per year.
Also as others have said, your spending thousands to get you into a car that's only 18 months newer and saves you a little on car tax. You wont save any money on fuel. You cars only 5 years old, not even half way through its realistic working life.