It costs £27.50 and you can order online and have it delivered in your size (ranging from 8 - 22) with FREE DELIVERY to your local M&S. Now when I picked mine up in store I was a little worried (excuse the pun) as I'm only 5ft4" and it's a TALL one size fits all type garment. But somehow it works. The onesie is lightly elasticated at the waist and ankles, so it doesn't sag or trail, even on tiny ladies like me. While I'm talking about it, I'm going to admit something that's probably not very cool, but I get all my jeans from M&S Denim Collection. They have a comprehensive sizing guide and three different leg lengths available for each dress size. So I can wear a 12/short and not have my jeans soaking up rain water, trailing along the pavement.
But back to onesies, check out the kids' and men's jammies they also have available. All non-bobbling, all soft and snuggly. Perhaps get everyone a new onesie for Christmas Eve night, then they'll all look smart in he Christmas Morning photos.