I've been tracking my fuel usage via fuelly.com I'm averaging around 40mpg and 370 miles per tank...and that's with around 8 - 10 full charge ups as well. I'm filling up when the reserve light comes up/last light on the fuel gauge, but only managing to get 41-42 litres into the 50 litre tank, so the range might be a little better, but I'm not brave enough to try it out! I've only filled up 3 times so fingers crossed this might improve a little.
C350e (Mercedes-Benz C350e Plug-In Hybrid) | Fuelly (Change the units to UK)
I tried an M4 run at 70mph ish in C mode and averaged 37mpg over 100 miles, and on the return leg tried E mode and saw 39mpg so not too much in it.