Holidays are not something that figure large in my life and we’ve been promising to visit to our friends Matthias and Sonja in the Ruhr valley for so long it had become a joke. As Matthias put it, “The kids would like you to come and stay before they leave home.” We flew to Dusseldorf airport; now I’m not a great fan of air travel – it’s not that I’m airsick or frightened – after all the terminal velocity of…