Not sure what year or miles yours had on it, but my 2011 DCT does considerably better than 21mpg. If I was doing those miles at that speed I would expect to return around 28/30mpg.
I average between 25/27 per tank on a 60 mile daily commute of mainly A/B roads with mixed 40 and NSL. A friend used to do the same trip in his 56 330i SE and used to get around 33/35 from what I recall.
With personal usage I still see 23mpg.
Jack
http://imageshack.us/photo/my-images/64/largenl.jpg/
Your DCT has 1 extra gear than my manual and the official figures from BMW have it around 3mpg higher on average than the manual. Plus I have an upgraded exhaust on mine, which may or may not make a difference.