Over 17000 migrants have arrived in the UK this year by crossing the English Channel in small boats - that is more than double last year's total. Both the UK and French authorities are talking tough, but the numbers keep rising. So why do two of the richest nations in the world find it so hard to control who crosses such a busy shipping lane? Lucy Williamson goes on patrol with the police tasked with turning the tide - and meets the migrants who are determined to outwit them.