In reply to andyjohnson0:
Having looked at my Rab jacket, I would imagine that it wouldn't be impossible to carefully unpick the stitching on the inside of the cuff, and either replace the elastic, or if that seems too complex, add more elastic and sew up the insides again.
If not unpick it, you might be able to to carefully cut open the fabric where the cuff meets the sleeve, and do the same thing. How happy you'd feel doing this might depend on your sewing skills.
Having looked at my cuffs a bit more, the actual fabric doesn't seem elasticated, with there being a loop of elastic inside the fabric, so you could probably just buy some more of the right width, and cut through the no longer elastic loops in your cuffs and replace with fresh elastic.
If any repairs you make are hidden from view, I wouldn't be too stressed about using a sharp craft knife to carefully open up the fabric on the insides of the cuff, you can always sew things back up again if you lose confidence and send it off for replacement elastic and nothing would be visible.
Post edited at 16:00