Returns Package Hasn't Been Collected For Nearly Two Weeks

A family member who lives overseas often uses my address to order clothes via their Next account (which uses Evri as their delivery courier), but the account contact details belong to them, e.g. phone number and name etc. - so it can be difficult to do any correspondence with customer support because I'm not the account holder. They recently bought me some shirts that were the wrong size so they arranged for them to be returned and refunded - which I'm unfamiliar with as a service, as I've only ever received deliveries from Evri.

However, every day for nearly two weeks, the family member has received an email stating 'Your courier is collecting your Next parcel today.' - along with a time range, but no one has turned up and collected the package from my porch. I don't even know what the actual tracking number is, because when I used the code that was on the returns label - there's no result, a seemingly non-existent order, yet surely they seem to be able to 'see' it on their system if they are arranging daily collections of it?

I'm at a complete loss as to what to do, as I've never used Evri to collect a package from my address before - I've been on the phone almost every day to Next customer support but get the same response every time that 'we will organise a courier to come tomorrow', which they never do - rinse and repeat every 24 hours.

Does anyone have any experience of returns gone wrong before? How do I sort this out? The refund is worth a lot of money and the return/refund window is nearly up (already halfway through the 28 day window).