hello i own a hotpoint ffa90 fridge freezer, the fridge is working fine but now and again i get a message on digital display stating that there is atemperature change in freezer and an alarm sounds the temperature change is 0 degrees , but when i put it on fast freeze this seems to help, but a few hours later it will happen again. i have noticed recently that there has been a build up of ice in botttom compartment , please help