What, exactly, is wrong with my fridge?

[snickersnack]

Was here when we moved in so no manual. After behaving perfectly for 4 years, I've just seen that the back inside wall is covered (and I mean covered) in ice and frost - it's like the freezer. Never come across anything like it before - and can't find anything because all the stuff at the back is covered in ice.

Any insights or suggestions very welcome...

it doesn't have automatic defrost?

(I think) Your fridge temp is too low. You should turn it off and defrost it.

If the door soesn;t quite shut propertly, then the fridge will have to keep "on" to keep the fridge at the right temp, this causes ice/frosting at the back (at least I think so, I just have to make sure the door is firmly sealed + the frost will go away afetr half a day)

Is the back clear ? If anything is blocking the air circualtion behind the hot air can't escape from the coil and it overworks.

These are all excellent suggestions and worthy of further investigation. The seal is a bit dodgy, so will have a look at that and adjust the temp as well.