You need to do about 20K miles for a diesel to be financially viable. They cost more to buy and diesel is more expensive.
A golf 170bhp from new priced at £23,000 compared to a GTI priced at list of £19,995
Volkswagen Golf Hatchback
2.0 TDi (170bhp) GTD 3d DSG
List price £23,160
Fuel consumption 50 mpg
Price per litre 104.5p
Extra cost to buy £3,165
Saving per 1000 miles £15.61
Miles to break even 202,781
Time to break even
at 12,000 miles per year 17 years