I agree with mac7 but will develop this a bit further.
Firstly, the GTD is a slower and cheaper car than the GTi. If this Diesel 'GTD' had equal or better performance, it would usually command a premium price over a GTi. (You will notice most Diesels are more expensive than their petrol equivalents).
Therefore, for a private buyer like me doing approx 7k miles per annum, an EQUIVALENT Diesel makes a poor case for itself. The final end of year costs would probably be similar for these EQUIVALENTS and for my low annual mileage. However, by choosing petrol I feel I get a more refined car that suits me more.