Cash Crusaders Hatfield terrible service

So I bought a phone from Cash Crusaders Hatfield, an iPhone 12 mini, 3 months after, the phone decided to switch off and did not want to switch on, it was still in perfect condition so I was shocked to see it off. I had the receipt for it for a 6 month warranty then returned the phone for it to be fixed or a new one. They told me it would take 21 days to fix it then I gladly agreed, they've been delaying the process for more than 21 days, and every time i call them, they don't respond or seem to take my case seriously, and I'm wasting money here trying to call them but their not picking. This a terrible service at its utmost and I honestly wish for this to be fixed ASAP.