Gas problem with Samsung American style fridge freezer

Hi - we called out an approved Samsung service engineer today to examine our RSG5DURS1 unit that we've had about 8 years. The freezer compartment has not been freezing properly for some time and we notice problems when the ice compartment is being used. The engineer examined the back of the unit and said the compressor was very hot. After he took all the food out of the freezer section and opened a panel at the rear, I could see the upper third of the coils were iced up as expected but the bottom two thirds were not. He said the problem was a lack of gas and that it would have to be topped up (with no guarantee that the problem would be sorted) and that a systems engineer (from Newcastle) would have to be instructed to do the work as he wasn't qualified to do it. I live near Glasgow so it seems strange that their nearest engineer is in the north east of England! No indication of the price was given but I'm guessing it won't be cheap. Is this a common problem with Samsung freezers? Is there an alternative explanation, another way of fixing this problem? Or will we have to consider buying a new fridge freezer?