The Thudbuster suspension seatpost is available direct from the manufacturer. They have many rave reviews and are the leading suspension seatpost. They are in the states but will ship internationally. They make 2 seatposts to fit the Dahon seatpost tube. You need to decide whether to go for the short travel or long travel version. The long travel version will provide a better effective suspension at the expense of having a bigger folded package (both systems will increase the height of the folded bike). You should order the following: 33.9 x 510mm (LT model) or 33.9 x 460mm (ST model). Shipping costs are $39.50 (they say to expect 5 days) and the posts themselves cost $139 (both LT and ST) so your total cost will be $178.50 (about £93). Theoretically you should also pay VAT and possibly import duties - VAT would bring it up to £109.28 and, if I understand it correctly, import duty, charged at 3.5% would be below the £7 minimum and thus would be waived - but I may have interpreted this wrongly - see this website for details: http://tinyurl.com/5b4873). The Thudbuster website is here: http://www.thudbuster.com/