Dee

Great tip. Failing that, splitting a strand from a thicker yarn in the same colour family is usually closer than buying matching weight, since exact colour matters more than exact thickness at 5mm.

Stabilise first, always. Get the jumper flat and dry, then run a line of small running stitches in a fine matching thread in a square around each hole, about 5-6mm out from the edge, catching every loop as you go. That fence stops the knit from running while you work. Only then start the actual repair. On fine gauge merino I'd then do a duplicate stitch repair rather than a woven darn, following the path of the original stitches with a single strand of matching wool, so the mend disappears rather than sits on top.

For heels specifically I've switched to swiss darning over the thin area before it becomes a hole, and I only weave when there's an actual gap. Duplicate stitch follows the knit so it stretches with the sock and you basically can't feel it. Needs catching early, which is annoying advice for a sock that's already got a hole.

Staggering is the step most tutorials skip. It also stops the patch tearing away at the join, which is the other classic failure.

Before you swap it, run the zip closed and look closely at the coil under a lamp. Worn or flattened coil sections give the same symptom and no slider will fix that. If you can see the coil is squashed or the individual loops look uneven in one stretch, you're replacing the whole zip.

If you do this, match the seam to the existing flat felled seam style on the outseam or it'll read as a mistake rather than a choice.

Elbows also just feel different from anywhere else because you notice them constantly for a week and then never again. I've redone perfectly good mends because of that feeling. Give it four wears before you judge.