Why haven't I discovered these before now!?
They are baby vest extenders.
They add much-needed inches to any baby vest which is getting a bit snug length-ways. This is especially helpful if your baby wears bulky reusable nappies, or if you like to get as much wear as possible out of an item of clothing before passing it on.
These (three for £7, plus £3.10 delivery) are a good-quality version from a respected UK nappy website, so the fixing mechanisms are top-notch and they don't seem to shrink in the wash (I believe cheaper versions are available). These one-size-fits-all vest extenders from The Nappy Lady website, attach to pretty much any UK brand of baby vest. They are delivered in a small plastic bag (sigh) inside a plastic-free postal bag (yay).
I wish I'd found these sooner, but I'm glad I've found them now.