If you use these 1800 20c batteries you will probably be better off getting four of a similar size that you can use in other aircraft and double them up as Gareth suggested. I recently purchased 4 flight max 2200ma 3s 20c from hobby city for about $11 each with postal cost of $26 total of $70 or about £45 at current exchange rates. We were talking of a 40a max requirement, but in reality you will probably fly your twin for most of the time on less than 20a draw, so they will be ideal. They are also a usefull size for other small to medium size single motor aircraft.
Phil