My gauge takes about 4 miles to start a reading, and as you say about 10 miles before any real heat is evident. I have decided to put it back into the dealership to check it out as Im pretty sure that mine is not right. Hopefully they will be able to sort it, the only problem is if they don't think there is a problem,,, they will bill me for the time :-(
PTC heater, that's a new one on me. Reading up a bit I can now see that there is a problem with modern efficient diesel warm up's. That's thrown a spanner in the works here,,,,.