Allotments 4 All

Today three of my 2ft high sunflowers were beheaded! Any guess as to the culprits? We have lots of wood pigeons around and slugs and snails but I would be surprised that the latter two could cut through a 1cm stalk. Rabbits and deer are out as we live in suburban Birmingham!

Don't rule out slugs and snails as they have beheaded my sunflowers before. Could it be human little pests?