Double direct debits

Changed and reduced my policy to pay annually and they continued to bill monthly for 9 years on my original policy as well! Now they want to try to fob me off with refunding the agreed annual fee and not the higher monthly premium which was supposed to be cancelled- playing dumb, it’s been going on for months and refused to put anything in writing “it’s not their policy to put it in writing” who on earth does business only verbally ??