Given there is no legal mechanism to force any government to stick to its manifesto commitments it's arguable that no mandate is necessary for any action unless it was either very actively campaigned on by the party in question or is very obviously outside that party's normal policies. As I mentioned in the ministers thread the reality in this country is that public transport is not a political priority for the majority of both voters and politicians themselves. This is not Switzerland or the Netherlands!
It's true that manifestos are non-legally binding.
I don't think that means that voters don't have a right to be aggrieved when manifesto pledges are blatantly disregarded.