Done a few sums based on 23000miles with petrol at ?1.37 and diesel ?1.43 (averages from petrolprices.com)
180 - 25mpg - 920 Gallons - ?5729.89
120 - 30mpg - 767 Gallons - ?4774.91 - Saving ?954.98 (?1637.11 If you can squeeze 35mpg out of it)
115 - 45mpg - 511 Gallons - ?3322.69 - Saving ?2407.21 (?2739.47 If you can make 50mpg)
If your after saving money and plan to keep it for a while the extra cost of the diesel would soon pay for itself.
The 115 cuts your fuel bill by 42%
Im tempted to get one myself now!