2006 1.4 fuel consumption

No petrol leaks/smell?
How does the engine perform? Sluggish or like normal?
Remove the spark plugs and check for black sooty deposits/perhaps petrol smell.
It's possible the up stream O2 sensor is faulty.
You can remove the two O2 sensors and swap them. Remember to put green in blue connector and blue in green connector. Disconnect the connectors BEFORE unscrewing the sensors.

Jjb0800 wrote: ↑Wed Dec 09, 2020 7:28 pm Just checked the mpg again and its worsened! Im getting 22mpg and also fully tanked it to 39.5 litres and reading on dashboard said 305 miles, surely there is something wrong??

If you are saying that you did 305 miles and used 39.5 litres, that calculates as 35mpg. Not great but better than the computer generated 22mpg. It would appear that there is some kind of fault with the computer calculations, and a second, possibly unrelated fault causing increased consumption. It may not be relevant, but we had a 2008 Clio with a trip computer (diesel) which used to read mid 60s mpg and only achieved high 50s by filling. The reason for this was that the computer reading for consumed fuel would be 10 gallons, but the actual fill was 11 gallons, and adjusting the computer reading by 10/11 gave matching figures. Although how the computer came up with those figures I have no idea.

If the 02 sensors are faulty would i get an engine management light?

It depends on the fault. If it's open or short circuit probably. I say probably because not every fault will cause the check engine light to come on.

I am more talking about deterioration of the O2 sensor's performance and not outright failure.

The O2 sensor puts out a voltage range of approximately 1 volt. 0 volts being lean, 1 volt being rich and 0.5 volt being ideal. Those are approximate values, in reality they are slightly different. The engine ECU uses this voltage to see if the engine is running the correct air/fuel mixture and adjusts the injector timing accordingly. If the voltage being out put from the sensor is not correct (faulty sensor) then the engine ECU will not be injecting the correct amount of fuel. The engine ECU can only go by what the sensor says.

I don't know of any way to calibrate the sensor, that is, check that it is putting out a correct value. Its easy enough to check if it's working but I don't know of any way to see if that voltage is correct.

"Experts" say the upstream O2 sensor(s) should be changed from time to time because they can deteriorate over time. However...

The O2 sensor is a critical part of the engine management system. You'd think that if it was so important and it deteriorated over time, that there would be a schedule to replace it, but there isn't one!
I don't know of any manufacturer that recommends a service life for O2 sensors. So you'd think, for the most part, that the O2 sensors don't deteriorate that much over it's intended life?

35 MPG is not bad for city driving.

The engine management unit doesn't read the actual flow of fuel into the engine. It guesses the amount used by calculating the theoretical volume coming out of the injectors from each pulse and multiplying that by the number of pulses per distance traveled. Not perfect but 'good enough'.

I don't know where the trip computer data is held my guess is the BSI. If I remember, this data is volatile, that is, it's lost when the battery is disconnected. In my car it resets to 30l/100Km. Is it possible that the data is being reset by a loss of power?