Hi Duncan,
Selling the funky Coupe to drive a car that is devoid of driving enjoyment is a Father's sacrifice. You have brought a tear to the eyes of the Used Car Expert team.
Forget the BMW, your budget will buy a banger with huge mileage that will be very expensive to repair.
The Superb makes sense as a nearly new car, but the depreciation is not heavy, so you end up paying quite a lot for a very basic used car.
You could buy a 2002 1.8 petrol but, I would go for a Passat of about the same age. You get a better interior and more comfort. It is also a quieter ride (more because of the sound proofing than the engine).
However the things against the Passat are MPG (even with the diesel) and repair costs.
The Mondeo - again same age - will be cheaper to repair, and give you nearer your 50 MPG target (48 MPG).
But the car you need to consider is the Peugeot 406 2.0 HDi. It is a bit cheaper to buy, pretty cheap to keep on the road and will give you the 50 MPG. The seats are lovely and soft and the ride is very cosseting.
Matt