Is the EML on the dash... or were you just scanning for codes? I've had P0135 logged since I bought the car a year ago. The oxygen sensor has a heater element to
help it warm up quicker. With a fault in the heater element, means it takes longer to get up to temp and burns more fuel. But at £135 I was quoted just for the sensor, that's going to take a lot of wasted fuel to make that price. As for 1180... is it petrol or diesel, how many miles on the clock and when did you last change the fuel filter?