Faulty Kitchen Appliance - Retailer Won't Refund or Replace

I bought an expensive range cooker eight months ago that stopped heating properly after six months. The retailer is refusing to replace or refund it, claiming I'm outside their returns window. I have all receipts and correspondence. The product clearly has a manufacturing defect. I need someone to review my consumer rights under UK law and advise whether I can force a resolution or pursue a claim. This is frustrating and I want proper legal guidance before taking next steps.