Thought this might be useful to others making the move.
When the chap from Crown came round to give us a quote, he suggested we got vacuum bags to compress bedding like duvets and pillows. That way they take up far less space in the container.
I checked around and M&S do them, but there's some cheap and cheerful ones from Dunelm which are £5.99 for two (one enormous, one just large). Once we got the vacuum cleaner onto them, they reduced our duvets quite impressively. Just hoping this gets us into a 20' container without the predicted overflow.
Jos