Even with no battery left it's still better on motorways than my previous car. I get around 42mpg in the PHEV, but only used to get about 30mpg in my Subaru (I suppose that big 3.0 flat six engine did encourage more overtaking though). It's a big, heavy car so never going to be great, even with the engine working in a restricted efficient rpm range and using fairly low loss transmission (compared to traditional mechanical 4WD).
So for weeks where I'm only commuting to the railway station, I can do the whole week on about £2 of electricity, with virtually no petrol (just for demisting the windscreen before heading home - can't pre-heat in the station carpark). As the weather's warming up, even that little bit will stop soon.
I think that it's more reasonable to compare pence per mile (or cents per km), because a tank of petrol could last weeks or months if you only do short trips, or less than a day if you do lots of long ones so mpg or l/km can be fairly meaningless on a plug-in hybrid. But that makes it difficult to compare different models, hence the standardised tests to give an advertised figure for new cars - that may or may not be acheivable in real use.