Fox, LOL.. I like the passion you put into your posts, which is a good thing..
I'm afraid that you are just 'wrong' in a lot of your 'facts', it's not a personal attack, just trying to reign in your enthusiasm that may cause someone to incorrectly make a decision..
At the end of the day, the OP has a 6K Budget, so he can buy a 6K diesel or 6K petrol, the fact is, that it is recognised that the diesel model will depreciate less, and therefore he will recoup more of his 6K when he sells the car..
Of course, it's not as easy as that.. go and do an Autotrader search on Ford Mondeo, Max Price £6K, Price High->Low, with GHIA as a search term..
It seems there is no difference in Petrol to Diesel models at this price....
Also he has saved some fuel money, and without knowing his driving style or annual mileage, we can only guess, I can make up some numbers too, based on my best guess from my experiences of having them as hire cars, I'd say drivng a little more conservative then I do, it would be 28 and 42 MPG (85 MPH Cruising + 10% heavy town driving, ), with petrol @ 89.9 and diesel @ 91.9p/litre that's.. £1746 Vs £1192 yearly fuel bills, a saving of £554 pounds a year... now that's me of course, and my driving style, and I do much more then 12K miles per year.. but it shows that it's difficult to quote fuel savings unless you have much more information..
On the subject of new cars, Autoexpress had a good aritcle on Diesel Vs Petrol, and you seem to be largely out of touch with the 'real world'. it seems that there are one or two examples that are as bad as you claim, but there are more that make a much more convincing case, the VX 1.9 Zafira was one such example, not only was it easier to drive, being a diesel, but it worked out much cheaper over 3 years..
It seems that in the majority of cases, it is not a false economy.. I wish I could find the article online, as it may be an eye opener..