Poor service on returning an air fryer

I purchased a Milex air fryer through One Day only on the 25th of November and received the item promptly. The exterior of the device got extremely hot, to the point where you could not touch the buttons. I contacted Milex, who advised that I needed to organise a return through One Day Only.

I arranged for the return on the 9th of December and they sent a courier to collect the device. It is now the 8th of January and I have received no response as to when I will receive my refund or even that they received the fryer from the courier.

I have sent them at least 10 emails, to which I just get an automated response stating that they will get back to me in a day. This is absolutely ridiculous. I can't even find a different way to communicate with them, like via telephone.