Adam,
Get her on a CO machine to analyse the emissions, expect <1.5% but nominally around 0.4.
All my P8 software is closed loop and i would hazzard a guess that your lambda sensor is outputting a constant lean trigger voltage and she is running around 10% at all times.

If this is teh case, you will need to perform a P8 reset after the sensor is renewed, you may be best to call me for details on how to do this.

Most P8 owners on 803s T34 report around 25 - 30mpg taking it easy with my software, you have a definate fault there unless your tanks are all used at silverstone...roflol