i've just found out that you're supposed to turn your work each time you start a new row. oops. i just carried on merrily around, but i'll bet it counts as some kind of special technique with its own proper name :P
think i've also worked out why the line where i joined rows was staggered diagonally instead of straight up. i was skipping the first stitch and going into the second instead. presumably i also made an extra stitch at the end of the row so i didn't actually decrease at all.