Richard,
I feel that you are going to have to stick with your current car as financially, it will not stack up for you to change to a diesel.
A dealer will buy your car from you, however, they will want to pay you somewhere in the region of £6000. I assume you will have paid somewhere in the region of £8000-8500 9 weeks ago?
At 10-12000 miles per year, it is borderline whether it would be worth you having a diesel car anyway. The cars are generally about a thousand pounds more expensive and the fuel is 7-10pence more expensive to buy from the pumps. Take into consideration the amount of money you will have lost already if you trade you car in now and think long and hard before becoming a car salesman's delight.
A diesel car will have you spending less time in the fuel station - but have a look at the overall cost of changing to and running a diesel.
My advice, stick with what you have for a while - probably not what you wanted to hear but there it is.
JJ
You gotta tie yourself to the mast my friend, and the storm will end.