I had the same problem with Comet some years ago.
We bought a Microwave with a built in oven.
I left it in the box for 5 weeks and we had a new kitchen fitted.
On first use , it burnt a whole in the microwave door and I had to unplug it quick and chuck a glass of water over it to put out the flame.
Comet then insisted they would not change it because it was more than a month old and would only repair it.
I told them it was the first time it was used and dangerous in my view....
After several phone calls and arguments , I told them I would come to the store with the microwave and take a new one from the shelf.
That would be theft sir , they told me.
In that case I said , call the police and they can meet at your store at 5pm , when I swap the microwave.
30 minutes later I got a call from Comet head office and they said they would either refund me or swap the microwave.
Never used Comet again ever - Guess many also don't and thus why the firm is bust ...well worth £2 :-)