After reknitting the gauge swatch, and after machine washing and drying it, I re-measured - it's just about perfect for both stitch and row gauge! Phew! And, as an experienced knitter kindly pointed out, feet come in all shapes and sizes... maybe a little longer on the heel flap is not such a bad thing? So I've picked up the needles again and am carrying on with the pattern.
Looking on the bright side, now I know that even after machine washing and drying, my finished socks will still fit an adult foot. So I haven't completely wasted the last couple of days. And I'm starting to feel pretty positive about this sock knitting thing again.