mpg shot up over past few days

And even then, there is always some spare when you are done, so may well have made the 600 mark.
Fill the car to the neck as close to the 0 miles warning as you are comfortable with. See how many litres a full fill takes. If it was only 47 litres then you know you had at least another 3 litres left in you tank and another 30-40miles? Don't run out ;)
 
The tank at 50l seems to me rather on the small side..

There's always a bit of a trade off - smaller tank is lighter so better efficiency, and it means you can have more boot space (as that's typically where it is). Of course it means you're filling the car more often, but I don't think 600 miles on a tank is bad really - I get about 300 on my petrol 8P. ;)
 
Yes because of safety I think they put it in front of the rear axle these days...
 
Winter Diesel to BS EN590: 16th Nov to 15th March.

Summer Diesel to BS EN590: 16th March to 15th Nov.

Basically we have just switched over to summer diesel and this will be peoples first fill of summer fuel. From my experience the fuel additives quality and amount plays a big part in fuel performance.

Also, like others have said diesel engines are much more efficient than petrol engines because of the lower wasted energy (heat) losses. Diesels are typically 30-35% more efficient at turning it's fuel into mechanical energy than petrols.
 
Winter Diesel to BS EN590: 16th Nov to 15th March.

Summer Diesel to BS EN590: 16th March to 15th Nov.

Basically we have just switched over to summer diesel and this will be peoples first fill of summer fuel. From my experience the fuel additives quality and amount plays a big part in fuel performance.

Also, like others have said diesel engines are much more efficient than petrol engines because of the lower wasted energy (heat) losses. Diesels are typically 30-35% more efficient at turning it's fuel into mechanical energy than petrols.

Summer diesel will go down to -5c and winter to -15, not that we've really seen -15 here in the UK this year. But then we all don't run diesels do we....