It is heavier yes, by about 200kg, so a couple of large passengers (although it feels a lot more than that behind the wheel, my 3 feels lighter to drive), but also a more advanced engine (I'd assume, the Polo was a 2000 plate so lots of time for engine development) and the Focus is driven more gently than the Polo.

If that's what it is, then that's fine, it's just a little lower than they expected when buying the car a few years ago, and I thought the 'stat might be a culprit after having noticed odd gauge position and seemingly low running temp.