agree that you should have probably taken them back before fixing them, and argued "not of merchantable quality", but they might see you having tried to fix them as a reason not to refund.
However. You could say you bought them for a specific function, and repaired them temporarily as you had no time to change when you realised they were coming down, and say you want to keep them, but would appreciate the cost of professional repair, or a replacement.
Next are usually quite good. I took something back (baby clothes) because I was cross the smocking had started to unravel, and although it had clearly been worn (slight staining around the neck area) they refunded no quibble.