What did the grandfather use to have?

My grandfather used to have a beautiful gold pocket watch. He wore it on a fine gold chain across the front of his waistcoat, and when I was small he promised to leave it to me in his will.
   "When I’ m gone," he said, "this is going to be yours."
   Unfortunately that will never happen now. About three months ago, my grandfather came up to London to visit us. The first Sunday morning after he arrived, my youngest son said he wanted to go to the park.
   "We’ll do better than that," said my grandfather, "we’ll go and feed the pigeons in Trafalgar Square." So off they went. They got home about teatime and my grandfather was looking very upset.
   "My watch," he said, "it’ s gone.  Someone must have stolen it while we were feeding the pigeons."