Thanks ladies. I've decided to email the pics to customer services, and also have taken some that clearly show the label. Am ok with keeping them, as they'll do for later, but it's a bit of a pain because I thought I was all sorted for the next few months and now I'm not!
I also decided to measure each of the sleepsuits, and they're all different - of the smaller ones, I measured 2 as one is in the wash, and one was 58.5cm and the other 61.5cm. Of the bigger ones, two were 66.5cm and the other 70cm! I'd get the whole of Adam in those, head included...
Have never had trouble with Sainsbury's clothes before, I like their stuff because it's cheaper than Mothercare (£6.50 for 3 sleepsuits rather than £13.50) but just as good, IMO. Next time I'll take the things out of the packet first. And at least it's all too big not too small, that would be more of a pain!