So my commute is 11 miles both ways, so 22 miles a day. Its from south london into central london.
In the mornings, i do this commute around 530-6am, so i can hold a steady speed and theres no/little traffic meaning I can get about 21-23mpg MAX (been doing it for 4 months and 22.9mpg is my current record!)
On the way back in the evenings there is a lot more traffic, and so a lot of stop start...best I ever get there is 19mpg.
I've NEVER got higher than 22.9mpg!! :-D
I pay about £50 a week in petrol, although that does include some weekend driving, but if its 'spirited' driving you do on weekends (like you should!), then whatever petrol is left by friday night will be gone by saturday morning!
Hope that helps!