Food is freezing up in Hotpoint fridge

Hi,
This is my first post on here, so apologies if I do anything wrong.
I’ve got a frost free Hotpoint FFP187BG frige/freezer which has been absolutely fine for the last 3 or so years.
I have always had it set on the icare temperature setting however, over the last couple of months it has started freezing items in the fridge, especially in the salad drawers. The back of the fridge was also about half an inch thick with frost/ice. I turned the temp down to 1 to see if that would help.
I did a bit of research and thought it could be the defrost heater so I ordered a new one. When my other half took the back panel off inside the freezer, the evaporator was completely iced up. Once the ice had melted off he fitted the new defrost heater and we switched it back on to the icare setting. The following day, there was frost on the back of the fridge again and items have been freezing again.
Could it be the defrost timer? I’m no engineer, but understand that the defrost heater comes on intermittently controlled by a defrost timer. Is this correct? I can’t find a defrost timer for my particular model of fridge.

Can you help?

Thanks

Claire