I had an engineer out to solve exactly the same problem on the same make and model. It has now been deemed beyond economical repair (it's less than 2 years old!) and about to be replaced. Apparently as it is a frost free freezer, when it ices up on the coil the thermostat or whatever controls the perfect temperature has gone. This then stops the cool air travelling up to the fridge and therefore stops it working.
What you are doing is exactly what I did, turned it off and it worked fine once it defrosted, but as soon as it starts freezing again, the fridge stops. I also had the loud bangs which is apparently ice falling internally. Altogether, not too impressed with this particular fridge freezer!
Sorry to be the bearer of bad news.
Mrs MH Gt Yarmouth, January 2007