To add, I drove probably 2 miles, 2.5 maximum. Low speed high revs for half of it. Temperature outside was about 9 degrees. Even so, I've never noticed it warm up so quickly

The advice I was given from an owner when I got my first vRS is that the temperature needle should always be dead vertical (to within a gnat's) once the car has warmed up and if it's not or it changes when you're driving then there is a problem.

 

If it's going low, it sounds like you're getting too much cooling so one of the thermostats is almost certainly stuck, probably open. I think my engine (vRS, petrol, manual transmission) has two so your car probably has two or three as it's a DSG. The good news is that they seem to be cheap (online part-only prices seem to be in the £5-25 range, depending on brand)