how can i fix my fridge freezer?

i have a hoover hca 33 type pf34

frost free fridge freezer

the freezer section is fine but for months now we keep getting ice build up on the mid rear back wall of the fridge section. it can be removed but starts to build back up immediately. Its quite a large block of ice and seems strange to have that kind problem in the fridge area of the fridge freezer! can anyone advise solution to the above. i need to remove the ice once every 3 weeks or so and is sooooo annoying - at £500 for new one - hope there's a low cost solution