Bought a thermostat for my oven on Thursday 09 October 2025 at Lynnwood Domestic Services, fitted it, and at first it worked

Bought a thermostat for my oven on Thursday 09 October 2025 at Lynnwood Domestic Services, fitted it, and at first it worked. But then within 5 minutes it stopped working. After about 30 minutes it started working again. Called in the help of an electrician. Removed it again and electrician tested it with multi meter to confirm continuity, it worked sometimes and other times not. Electrician advised it is a factory dud, it happens, and because it is still brand new and to be returned within 24 hours, they should exchange it. I then returned it on Friday 10 October 2025 with the proof of receipt, but "Stefan" just refused, saying they do not give any guarantees on electrical components. It it was 6 months or a year down the line, I could've understood maybe, but not within 24 hours! Not satisfied with their excuse on not giving any exchange warranties on electrical components, since they are an electrical service supplier specialist.