If you open the door and look at the vent at the very top do you see a build up of ice or frost in there?
Ok what it will be is ice will have pooled up at the back of the evaporator housing
so this blocks up the fan and stops the air circulation that's why the bottom isn't as cold as the air isn't getting blown down there.
Now for this you need to empty the freezer and turn it off for 24 hours and leave the door open as this will allow the ice to melt.
Once defrost retry it and that should sort it.
If seen this fault a lot on these and the defrost nearly always cures it and gets it working again.
You need to do the defrost as its very hard to remove the evaporator cover on these when its iced up. You get so far and it wont move or it breaks the evaporator and then it needs to be regassed and a new evaporator so its a lot easyer and safer to defrost it for 24 hours and that should do the trick.
If you need anymore info just get back to me.
Yes the polystyrene on the bottom is normal as theres a base heater under there so don't worry about that that's normal and ment to be there.
Is it the freezer dial you have tried turning?
If so does it feel quite loose?
Ok these dials have a habbit of breaking in there so when you turn it its not actually turning inside.
Use a pair of pliers and pull the dial off as its just a push fit and you will see if it has
snapped as the other part will be left in the switch on the board.
If so use a pair of long noise pilers and turn it by hand very carefully off to reset the lights.
Then pull the rest of the shaft off and super glue the shaft back to the diald then you can refit the dial.
Just glue the shaft while its out and leave it to dry before refitting it and it will be fine and wont get stuck in the board.
Ok no worries you can just buy a new one anyway and push it in.
If its the silver dial then the part number is C00193778