Just to give you some idea although your figures depend how you drive, the harder you drive the less miles before a dci becomes economical,
Using the figures I'd get with petrol vs dci.
Dci about £1.1k more expensive (£1k list +£100 interest on loan over 3 yrs).
To do 10k in a petrol will cost about £1250
To do 10k in a dci will cost about £740
Saving in road fund over 3 yrs is £70
Therefore ever 10k you'll save just over £500.
Resale value of a dci is about £300 more than a equivalent petrol.
My figures arn't bias towards either car if anything I'm bias towards the petrol.
So assuming you'll do over 16k in 3 yrs the dci is more economical.
In my case I've done 54k in just over 3yrs so I've saved £1.7k minus what its worth extra.
Even the cost for the extended warenties is the same.
Servicing prices are the same.