Hotpoint Mistral FF92P Fridge Section Warm !! HELP !!

Hi all, this is a recurring problem which I thought was sorted. to cut a long story short, the fridge section wasn’t cooling, so I turned the thing off, stripped the cover off the freezer section & defrosted it with a hairdryer. ( after reading some of the posts )
It worked a treat, freezer perfect & fridge cool, however after about 3 weeks it started to give strange readings from the fridge section again (anything upto about +30 deg) & my missus swears it was warmer inside than the house, although I don’t see how that’s possible.
The freezer section appears to be working perfectly and hasn’t warmed at all ( although it has shown the occasional dogy warm reading) it mostly reads -22

How does the fridge section get it’s cooling? is it blown through from the freezer? All I can see is a vent at the top right corner of the fridge, but I’ve no idea how it works.
There is also a large unit on the left hand ceiling of the fridge that the light is built into, does that house a fan that circulates the fridge air? if so, could that be broken, or is that controlled by something else.
I don’t think excessive frost will be the problem this time as it hasn’t been back in use long enough.

I’ve seen a lot of posts about the THERMISTORS, if I replace those do you think it would solve the problem.

Sorry it’s a bit rambling but I thought I’d add as much info as possible.

Any help or advice would be greatly appreciated as this thing is driving me nuts

Thanks in advance

Ade