I bought a really good value one from Wilkinsons a few weeks ago, for use in my caravan.
It was vacuum-packed very tightly and the instructions did say to open it carefully and lay it out, and it would expand to normal thickness, and.......and it did !....bloody hell, it was really thick !
I think it was this one :::
http://www.wilko.com/duvets/wilko-anti-allergy-duvet-double/invt/0324825
If not, it was their cheapest type, white and yellow packaging...thoroughly recommend. Actually, I can recommend Wilko for just about everything !