And the backstitch started:
It still looks really dark even with 3371 instead of 310, but I'm not unpicking it. Maybe next time (not that I'm planning a next time) I'll go for a lighter shade again.
The other thing I've noticed with the backstitch is that it's highlighting where the stitches aren't sitting flat, usually where they have slipped one way or another with the fabric weave. I think I'm going to try and avoid doing cross stitch on 22ct hardanger again.