Some of us struggle to get into the 20's when it comes to mpg.
I could only get 700 miles out of one fill up if I filed the interior wiv petrol.
I feel your pain (managed an average of 24mpg from my first fill) but I like to consider the total cost as I find this the most accurate measure of the real cost...
I travel around 6 - 7,000 miles per year and paid £1,050 for my 2002 V6. It has had lots of cash spent on it in the past and the HG appears to be perfect - no issues at all (I'm also quite a sedentary driver and regularly check oil and coolant levels).
By my reckoning, 6,000 miles will cost me around £1,400. The same 6,000 miles in a diesel (assuming an average of 38mpg) would cost me around £970 - A difference of around £430 so all things being equal and assuming the same costs for servicing, oils etc, it would take me over three and a half years of ownership before I broke even which is fine however based on my past behaviour, I'm highly unlikely to keep it for three years before upgrading anyway.
Of course I haven't factored in resale values (I expect that diesels have a higher resale price) however based on the amount I paid, I expect that if I sold it in a year or two, provided that it had a current MOT and Tax, I should definately be able to recoup the original purchase price whereas I don't think I'd be able to say that had I spent the extra £1500 or so on a diesel model.
I'm not knocking diesels (no pun intended here), I've owned at least four diesel cars but I think that when choosing a car, people shouldn't necessarily go for a diesel just because they're more economical (as I have done in the past) or just go for a small car over a larger one because they'd be more economical either because fuel consumption doesn't necessarily reflect the total picture.
Andy.