So we've received two phone calls from the manufacturer, SlumberCorp. (And none from the retailer).
The first one, on Monday, was to ask for our address to see/pick up the mattress, though we'd left it with the retailer when we complained to them directly. The manufacturer said they can't do refunds if we've taken it home because it's an 'intimate product'. My father (who took the call) told her we wanted to deal with the retailer, not the manufacturer.
Then yesterday we got a second phone call. The manufacturer, having seen the mattress, told me the problem was minor, just "a couple of missed stitches" and that we aren't entitled to a refund, nor to deal with the retailer directly, under consumer law. She offered to 'upgrade' the fabric on the same mattress instead.
Now, there's a chance that might fix the problem, but her insisting it was just 'a couple of missed stitches' does not fill me with confidence about their standards.
It was rather irritating too, for her to be throwing consumer law at me, after flat out refusing a refund without even seeing the mattress, initially. It felt like she was trying to pressure us into accepting. I'm not sure why they're fighting so hard to not refund it. If I'd sold or produced something dodgy, I'd be very apologetic and be more concerned about the reputation of my product/company.
I just want to get a refund and be done with it. I think it is a major problem and I took numerous photos of the condition it was in before returning it. I seriously doubt anyone would knowingly buy a new mattress in this condition.
The retailer still has quite a few days left to get back to me based on the letter I sent her, but I don't think she will.