Ceb80 - Great choice with the Sandy's, you won't be disappointed. I started on XS Sandy's with XS airflows, I only bought small but when the baby was here they were totally massive so I quickly bought XS and we started on week 4, she wore them for about two months, maybe a bit more but she is tall and skinny. I've just started her on the Smalls now, she's 15 weeks, I couldn't get her into the XS but the S were too big for a while, strange!
I also have some Itti Bittis, she's in mediums and they fit perfectly, I would think she's about 12.5 - 13lb now. Itti's are seriously cute, but they are small, I wouldn't expect that they'd fit for the weight ranges that they suggest, certainly don't buy smalls as they'll be a waste.
Essie - Are you having more luck with your pop-ins now?