I'm considering a petrol. I should be able to afford it and with my driving style should still get 40mpg when I want to.

No, does not prove anything, are you still considering a petrol? 600 a week would see a big rise in cost would it not?
AndyVTR - i'd stick with the diesel if i were you. The petrol costs are a nightmare, and thats on 100 mile a day round trip, similar to yours....
300 miles on a full tank (£50) is just mad.

Did she leave you because you love your car more than her? I too have that problem!!