According to the December Structural Reform Plan published on the Number 10 website:

And the reason?
Happily it emerges that the delay is nothing to do with DfT as it is apparently 'for reasons outside of the Department's control'. A splendid explanation that all TOCs can now happily embrace to inform passengers when their own trains fail to turn up on time.Meanwhile, does anyone have a clue what 'has resulted in a delay to commercial close of May 2012' actually means?