Faulty Hotpint Fridge Freezer

Hi – I have a Hotpoint RFAA52S fridge freezer which has just started playing up.
No matter what setting I put the thermostat on the Fridge very rarely works – sometimes the back wall gets cold but not that much and then it seems to stop – freezer seems to be working and then stops freezing and ice melts and then goes cold again – but not as cold as I would expect. The light in the fridge works – its making a “running” noise.
Does this sound like a thermostat problem – which I’m confident I can replace myself – or does this sound more complicated involving an engineer call out , in which case I might just buy a new one.