But it turns out the best muslin squares are from Marks and Spencer. Two main reasons:
1) They are thicker and so prove a better defence especially if your baby is cursed with reflux. It didn't seem a lot of milk going down but it seems like loads when you see it again.
2) There is a proper hem on them. After 20 goes through the washing machine (that'll be about a month) the John Lewis ones start to fray and/or curl up.
There is a price difference. John Lewis: £8.50 for 6. M&S £10 for 5 or £18 for 10. But I think it's worth it. Get them here.