Kier has been quite clear that is not the oppositions duty to make alternative proposals, but it is its duty to challenge the government.
This is something he has stuck to, however, at some point he will need to produce a manifesto that he is willing to deliver, and this will be his mandate for when, or if, he forms a government.
I would not like to be the one to say that he has flip flopped entirely on his own personal manifestos (and those of his party) , that he refuses to answer challenging questions, or that he has lied, (although he has, just like Boris), but I am sure that once the election process starts he will be open an honest with the electorate so we know what we are voting for.