Peter is one of our arch pro-Europeans, of course, and the reason he wants a referendum is because he thinks the problems of the Eurozone will eventually be sorted out and we will face a stark future if we remain outside of it; as he puts it a Canada to the United States (though Canada seems to me to be doing just fine, but that is another story).
Intellectually, he has a point. By which I mean not that I agree with his views on the Euro, merely that he has a case when he says it is a big question that probably needs an answer from the country as a whole at some point.
It could be simply done. A three question referendum, in which we have to decide to join the Euro, stay as we are or leave altogether would certainly clear the air.
If this issue is starting to grip the likes of Peter Mandelson, there must surely be an argument for David Cameron to seize the initiative and agree to put it in his next manifesto. The Lib Dems would probably not accept any move to have a referendum in this parliament, so he could say that it was impossible to hold one until after the election and in doing so he might well neutralise the (increasingly feisty) UKIP vote, not just next time but for good.
But he'll only get the credit from the right-wing press if he gets there first…