I just totted things up across my 7 days of driving...
1 - London - Portsmouth
2 - (ferry to) Santander - Tarbes
3 - Tarbes - Arles
4 - Arles - Grasse
5 - Grasse - near Orvieto
6 - near Orvieto - Obernai
7 - Obernai - (via tunnel) London
2,238 miles/3,557 km
average of 19.5mpg
or 6.87 litres per 100km
Which when you consider the car was rust holes (added lightness!) is not great