Not quite because diesels hold a premium over petrols in the used car market, as 2nd hand owners can see they save money @ £350 a year.
It's swings and roundabouts, unless you're closing in on 20k a year, or need a derv for some reason (company car, towing etc...) then I recon a Tjet will be almost as cheap and almost certainly as fun, if not more fun to use.