Presume as HMRC didn't have a big enough slice of debt to block the original CVA I'm guessing they couldn't really do much to stop an amendment should the other creditors accept it.
That they weren't even informed, given that they do have a place on the creditors' committee, is staggering.
If they didn't know, it's because they weren't informed. All part of the strategy to reduce unsecured debt to zero I suspect.
What I find very odd is that this debt has been restructured in double-quick time and with no leak to the outside world. Were they all asked to keep quiet?