The stock seat-post that comes with the Raleigh Twenty is 28.6mm chromed steel. It's long enough for people up to about 175cm or 5'9" tall. Of course, this will also depend on what saddle you have on top.
Tranz-X offers a 28.6mm seat-post in 350mm length. Usually only available in black.
IRD (Interloc Racing Designs) makes the longest 28.6mm seat-post on the market at 425mm.