The technician has set the mixture a little rich to "help the car along a bit". Once I have resolved the intermittent fault, I will ask him to reset the mixture to it's correct proportion. I removed the air temperature sensor last night and it looks like it's damaged. It is shaped like a felt tip pen nib and covered in black plastic film. This has eroded in part to reveal the metal beneath. I have ordered this part today (£13.00), and it should be available for collection tomorrow. I understand that if this sensor is faulty or has become disconnected, then the duty cycle will run at around 80% and fixed, which will give very poor mpg. I have not checked my mpg carefully, but the car certainly gets through some petrol.