Why we won't have a third referendum on the terms of leaving the EUThe 1975 referendum was on single market membership, not EU membership.
There is no authority for EU membership and the associated demonstrated loss of national sovereignty that subsequently occurred, the possibility of which was denied by Edward Heath.
A referendum would be needed to get that authority, and that authority was not obtained in either the 1975 referendum or the 2016 referendum.
Prime Minister Cameron, doing his best (we assume) was unable to find terms on which to get voters to accept membership.
As voters never accepted EU membership and associated loss of sovereignty, our EU membership is illegitimate, and so the default position is therefore to not be a member.
It becomes ludicrous then to seek for national agreement of the terms under which we won't be a member; it is membership that is illegitimate, the non-membership does not need legitimising in the least.
How could a possible rejection of the specific initial terms of leaving be construed as implicit legitimisation of EU membership, in the face of the explicit rejection of membership?
Should we remain by default simply because a few mardy ministers or civil servants will persistently to a bad job and refuse to facilitate a "good deal" in order to continually fail to get national approval?
No, we should assume the legitimate position of non-membership, and give them that to work from.
This is why we shan't have a third referendum on the terms of leaving.
We might have a third referendum on whether and on what terms to re-join the EU, in a few dozen years or so.