are you doing short journeys?
I reset my meter last week and drove 79 miles from West Berks to near Northampton, a mixture of A34, M40 and country roads. Taking it easy I got 41mpg .I reset it again for the rest of the week, driving 4-6 miles twice a day and over the court of 5 days it dropped to 24.5mpg. Short trips don`t help, stop-go also (obviously).
On average mine returns 30-31 mpg most of the time using 98ron, this is what I get if I don`t reset if for 2+weeks doing different types of driving (motorway, urban, rural)
Do the obbious, check tyre pressures etc. Also it`s worth getting a fuel card of some sort. I`ve got a Shell drivers card which returns some dosh now and again in the form of discount coupons ( but not much). I`ve also picked up a Citi-Shell credit card recently that I intend to use for fuel only, this gives cashback of 6pence per litre for the first 60 days and then 3p a litrer after that. Every little help.....
I think it`ll be hard to sell a car of decent value at the moment - don`t get buggered! But if you must......