To warm engine or water 20C you need to take energy from somewhere. In the case of idling diesel this energy is just not there. A 6 cylinder engine full of 3 gallons coolant has a huge thermal inertia and this kind of fast warm-ups and cool downs are not plausible. My opinion is that "London Lad" is getting a false reading due to the tool that he is using or due to electronic problem in his car.