Faulty Defy Appliance

I bought a new defy over and under fridge/freezer in Sept.2019. Some 2 months ago the temperature in the fridge compartment started to rise to 12 degrees and the compressor did not switch off. The fridge was still under guarantee. I reported the problem and a approved defy technician reported it is an internal gas leak and the appliance is beyond repairs. Defy refuse to replace the product. Their excuse is not acceptable to me but several attempts to contact them was unsuccessful. My argument is that it is a manufacturing fault and had nothing to do with the fact that an unapproved technician has worked on the appliance and therefore, according to a junior staff member, void the guarantee , I would expect re-consideration from senior member of the company but after to weeks they didn't even bother to return my telephone calls. The reason for getting a local technician to assist was that we were expecting visitors and was desperate to get the appliance up and running again. (by the way ,their approved technician took 5 days to pitch up) The bottom line is that there was a problem with the appliance which got nothing to do with the fact that somebody had fiddled with it. Only an excuse not to take responsibility for their substandard product.