Welcome Andy,
I have the same car - with less distance traveled (~250,000km).
Although I know this isn't your question, your consumption figures seem about right - maybe a tad high but that may be down to tyres, load and your driving style. I get 10-11l/100km around town and on flat, straight highway runs can get below 7l/100km (best I've had is 6.7l/100kms - nearly 1,400kms on a tank!).
I'm not 100% certain how the OBC exactly computes the consumption data but it has to be fuel used over distance (obviously!). It would be worth having a look at few values in the "unlocked" BC (assuming you have a high cluster with the text display).
First to check is the reported momentary consumption in the BC - menu #4. 4.0 will show the momentary l/100km value and 4.1 will show the momentary l/h value. Check these when driving at a constant speed and see if they match the gauge on the dash and are even sane numbers.
Next you can check test #8 - momentary road speed and verify that is sane also.
I think menu #7.4 also shows vehicle speed - not sure if it momentary or averaged.
Let us know what you find...
