Well there were only 4 people involved in the decision to drop the proposal: David Cameron, George Osborne, Nick Clegg and Danny Alexander. So it would take at least 3 to make the decision so I'd say Cameron, Osbourne and Alexander but I have a sneaking suspicion that Clegg isnt blameless either and that this was set up to fail from the start. When Clegg put through the original proposal back in 2011 the general consensus was that the criteria for recall was too high but he refused to compromise on it.