Okay here is the solution:
After the battle (at least according to the story) the Prince wandered over the battlefield and found the dead blind Prince who, when he heart that the favour of the battle was turning against the French, joined it, even though not being able to see anything. The Black Prince was impressed by his courage and decided to use his motto in his own badge. This motto was "ich dien" (German for I serve). Until today it can be found on the badge of the Prince of Wales:
http://upload.wikimedia.org/wikipedi...e_of_Wales.svg
The same badge was supposed to be used as the logo for the Welsh Rugby Union until the 1990s when the WRU changed it in order to be able to get a copyright on it they changed it into the form of the picture posted earlier and replaced "Ich dien" with WRU.