ELECTROLUX FULLY INTEGRATED 50/50 FF ERN2820

I bought this appliance in March 2005, no longer under guarantee or warranty. Has recently developed a fault whereby frost accumulates at the back of the fridge, quite badly. Tried defrosting but soon re-appears after switching back on. Got an appliance engineer to take a look at it. Said that the problem was with the gas and that he wasn’t prepared to investigate further as it might jeopardise any claim I might have against Electrolux. He also said that he would expect the life of a f/f to be between 5-10 years. If this is so, what chance do I have of getting it repaired/replaced? Preferably free of charge?!