I have decided to order a pair of small ones and ship to Germany where I will pick them up on weekend of 4th/5th November to bring back to London. Shipping will be Euro 3.90 then opposed to Euro 11. Anyone interested, I'd be willing to put in a bigger order.
http://www.singlespeedshop.com/Single-Speed---Fixed-Gear/Chain-Tensioner/Batavus-Chain-Tensioner---Chain-Tensioner-Pair.html
Seen this for much cheapness. perhaps a group order might be a good idea.