how much petrol(cost)

[simon.rowden]

hiya.i have a 1.2 fiesta petrol,i am going to chessington from witham essex about 78 miles leaving around 7am rushhour therefor i will be hitting the m25.need to be there at 9am so will hit traffic.does anyone have any idea how much this will cost me in petrol(round trip)heading back around 5pm rushhour again.