The EU Council President, Donald Tusk, has indicated the Council would be happy to accept a UK change of mind, and allow the A50 notification to be withdrawn. A50, itself, makes no explicit provision for this, and so we are in uncharted territory. My reading is that there is no legal right no withdraw, but the Council can make a political decision to accept a withdrawal.  As you say I think this would require unanimity, as it is similar to an indefinite extension of A50, which also requires unanimity.

However for this whole scenario to be plausible, a Brexit deal of some sort would have to be agreed in October, voted down by the Commons in November, followed by an election and change of government in December.  Renewed negotiations in January/February followed by a second referendum in March with a confirmed result in before 29th, March in time for the A50 notification to be withdrawn.

And as you say, it would take only one country to with=hold consent. A lot of ifs and buts. Very much an outlier scenario.

Index of Frank's Diaries