In 1948, Alderman Fred Henderson, a former mayor of Norwich who had been imprisoned in the Castle for his part in the food riots of 1885, proposed a memorial to Kett. Originally hoping for a statue, he settled for a plaque on the walls of Norwich Castle engraved with his words and unveiled in 1949, 400 years after the rebellion. In the 21st century the death of Kett is still remembered by the people of Norwich. On 7 December 2011, the anniversary of his death, a memorial march by members of Norwich Occupy and Norwich Green Party took place and a wreath was laid by the gates of Norwich Castle.. How many years was Alderman Fred Henderson imprisoned before proposing the memorial to Kett?