Would it affect my case if i messaged the seller, before sending the letter, asking if he would be willing to just replace the cooker with a functioning one or give me a full refund?

i bought an electric cooker Thursday 7th December from Facebook marketplace. The company had 5 star reviews and seems to have a steady flow of stock to sell. The cooker was delivered and installed at 8pm that night - everything seemed fine. I get home from work at 11pm and decided to cook some sausages. The oven had been on for 10 mins and the glass panel on the oven door completely shattered. After taking a closer look it is riddled with rust. I have the invoice receipt (i paid £170 in cash). I have an 8 year old son who has learning challenges and is currently being tested for autism. I contacted the seller asking for someone to come and collect the cooker asap and asking for a full refund, to be told no. He has stated because my partner saw it working when it was installed, he believes we have tampered with it (we havent). The engineer literally turned it on and off again (he also failed to turn the fuse box off before installing, my partner had to do this). The seller has stated because i reported the damages 3 hours after it was installed, he wont refund me. He has offered to 'try' and find a new door at a cost of £20 which i would have to pay. The cooker is supposed to have a 6 month warranty. After questioning the seller some more, he told me he only received the cooker that morning, after going through previous communications with him, the cooker delivered wasnt actually the one we had chosen. Is the seller in the wrong and can i get my money back?