Zanussi ZFU25200WA not getting cold enough, frost inside but its frost free freezer

filled up the freezer yesterday with a lot of fresh meat and pressed the fast freeze button like i normally do. in the morning i had a alarm flashing saying that the temp was 2 high, around -8. pressed the quick freeze again for a few hours but didnt change anything.. i took the top cover off the vent inside and saw that i had some ice there. i also blew into the drain pipe at the back but it seems to be completely blocked, should i be able to blow through it?. ive got the freezer off at the moment to see if it will sort itself out but im worried that the ventilator fan could have gone?
ive tried taking the plastic cover off from the top of the freeze on the inside but it seems to want to bring the defrost heater off with it so i have no idea how to get to the ventilator heater. any ideas?