By-elections are ultimately that- by-elections. They have their own special circumstances and their own issues. People have a tendency to vote differently than they would if the Government is at stake.
So, I'm not drawing too many conclusions from the narrow Conservative victory in Bromley and the Labour defeat in Blaenau Gwent (apologies for the mispelling earlier, BTW).
Labour got the majority down, but lost votes. It's more a case of people deserting the Indie candidate.
As for Bromley, Cameron can't call this a victory, but the Lib Dems can't either. It's at the end of the day, a by-election.
Labour's majority stays at 64/70 (if you count the SDLP).