I test drove a 55 plate that done exact same as you've said. I thought the cars must be that slow unti l drove the 1 I have now. Possibly not looked after or driven corectly? Cars made in the last 15 years or so, actually need to be driven quite hard from time to time, allot people drive diesels just about ticking over, which over say 100'000 is no good. Plus being run like that will make it slower than 1 that has been driven harder more often.
When i bought a fiesta st brand new, I asked ford on how to run it in, they said ''you dont, best thing to do is drive it like you stole it if you want it to perform like it should''.