Ross, The new national hive from MB should be ready very soon - it is supposed to ship from the supplier at the end of this week. Initially, the only way to make a 14 * 12 would be to use 2 supers and a thin distance piece, a bit like a shallow eke. This will achieve the right size but obviously it is not ideal. It will be a matter of judging demand to see if a 14 * 12 complete body would be worthwhile. When the new hive arrives we will look at the idea of a distance piece - probably cut out of suitable sheet material - and look at offering it as an extra.
John
John