A big thanks for your response.
I've been calculating the mpg of all of my vehicles since 19.10.11. I normally fill my tanks up to the second click of (generally) the same pump at the same garage. I use fuelly.com and it's really to give me an early warning of something about to go wrong. Here's the graph for my 2.5 litre 944.
The big drop in mpg was when it was taken off the road for a bit of a re-build and was clearly just shunted about the workshop for a tankful.
It's just odd that the on board consumption things on my XJ-8 and even my Vel Satis gave readings that were broadly inline with proper maths, but the 75 is just having a laugh. It will be used for a daily 26 mile commute over 4 junctions of motorway with dual carriageway at both ends. I'd hoped that it would easily do between 25 & 30mpg.
Am I dreaming?