Under proportional representation, the overlapping policies are those which pass the House of Commons. If Labour and the Liberal Democrats are generally inclined towards the euro, representing a majority of elected members in a new PR House, then it will be accepted, as the people will have voted for that House.
Similarly, since the Lib Dems look to increased taxes and lower interest rates, while the Labour and Conservative Parties take the contrary view, that would not pass. That is democracy.
The present system whereby the Government party is said to have a mandate for absolutely everything in its manifesto is, frankly, absurd, given the fact that no party in my lifetime has ever gained a majority of votes cast.
M G PARSONS