It's not uncommon for the TDCi to only warm up when it's 'under load, i.e when you're going uphill or accelerating swiftly. I live on a hill and the needle barely budges until I go up a big hill on the way to work. And drops down a touch if I coast downhill.

Swap the 'stats, chuck some new coolant in and also consider a new fuel filter as that's a very wise move if you've recently purchased a diesel Mondeo.