They've just been put up on Meanfiddler:
http://www.meanfiddler.com/listings/just-announced/3123/extra-flight-of-the-conchords-tickets
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Flight Of The Conchords' extra UK dates
Watched them at the Apollo last night and they were superb. They did a mixture of songs from both series as well as 3 or 4 I hadn't heard before.
Adj Barker was ok as the support act although loads of people were still streaming in during his set which must have been tough.