There is obviously a problem with your car, Jez can get more out of his S-Class.
Nope, had it checked out, it has a full ford service history, since I have had it it's been very well looked after. I found a st170 forum and it just so happens that they all have shocking mpg. I managed to get just over 320 miles from a tank once, nearly all motorway. And it costs around £55 to £60 to fill it up. I make that around 27mpg driving like a nun. Shocking but true.