I don't believe DC ever offered an "in/out" referendum, though some Lib Dems suggested one.
I do remember DC saying we should have had a referendum on the Lisbon Treaty. That referendum would now be a nullity, since the Treaty was ratified and enacted before he became PM. It would be retrospective and the UK honours its international commitments which it has made.
The default Tory position on referenda has generally been that they are opposed to them. They joined the EEC without one in 1973. They opposed Labour's referendum on membership in 1975. They opposed the devolution referenda in the 1970s. They opposed the devolution referenda and the GLA/mayoralty referenda in the 1990s.
They supported the AV referendum in 2010 because they knew the result would be the one they wanted. They now say that any future EU treaty which would result in a change of the relationship, by more power transferring to the EU, should be subject to a referendum, and that any such future treaties should involve a renegotiation of terms for the UK.
With no such treaties on the cards for many years, the promise is not necessary.
Personally, I agree with the former Tory position, that referenda are pointless. We have seen in Ireland what happens when the public give the "wrong" answer. They are asked the question again until they give the "right" answer. Since a future parliament cannot be bound, a referendum has no lasting binding power and so is merely a pointless exercise in populism.
It is far better to put plans in manifestos and let the public vote in an ordinary election. Don't keep asking them what to do.