Prince Charles laughed as a well-wisher shouted to him, "Congratulations Sir, it's triplets!"
As the Prince walked over to say hello, John Dowson joked, "I heard it on the radio".
Mr Dowson said afterwards, "He just laughed, he was really jolly. Everybody's so excited in the country for him".
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The letter calls for an end to harassment, abuse, and impunity in a world "ripe for change".
Police say that anyone who sees Whitehouse should not approach him, but instead call them on 999 and quote incident 23 of 18 February.