Unfortunately, it didn't reset the trip computer: it put it into diagnostic test mode. I could cycle through all kinds of tests (including the frightening "instrument sweep" that made all the dials drop to zero and then swing back) and another one that turned on every icon my dashboard was capable of displaying. It showed all kinds of different numbers and acronyms. It did not, however, switch off.
We retrieved the instruction booklet from the boot, but it gave no indication of how to get into disgnostic mode, let alone how to leave it. Eventually, my daughter found a document on the Internet that informed us the only way was to turn off the ignition — not what you really want to hear when you're driving down the A1.
Rather than having enigmatic numbers telling me how healthy or otherwise the car was, I went through the test cycle until I found one that checked the display itself. I was then able to travel in the comfort of a blue screen of death until we found somewhere to stop for lunch.
Turning the car off and on again did work as advertised, but it would have been more useful if I could just have held the selection key down for 4 seconds or something instead...