I have a Neff Fridge Freezer model no K8525X1GB/01 FD8206?

The freezer compartment is small and works well but the back of the fridge ices up and becomes a large lump of ice blocking the draining hole and in turn flooding the salad crispers. It then ices up further and I find all my veg and salad frozen and ruined. I bought this fridge from John Lewis and paid £700 for it. Unfortunately this started happening after the 2 year warranty so I called out the Neff repair man who looked at it and told me to pour a small amount of hot water regularly down the drainage hole and that would stop the problem. He charged me £60 and it's as bad as ever if not worse. I don't want to call him out again and wondered if anyone could tell me what the problem is?