MICHAEL Moore today admits that, while the UK and Scottish Governments are working hard to secure a deal on the independence referendum, no one knows yet whether an agreement will be possible; challenges remain and "nothing is agreed until everything is agreed".

The Scottish Secretary - who will break off from his attendance at the Liberal Democrat conference in Brighton on Monday to travel to Edinburgh to have his third face-to-face meeting with Nicola Sturgeon, the Scottish Government's Deputy First Minister - has already made clear that a red line for the Coalition is having a single, straightforward yes/no question on the ballot paper.

"I don't think we can have a referendum on independence if we don't have a single question," he has told MPs. Whitehall insiders have made clear that if agreement is not reached, then Westminster organising its own poll is "an option".

From his desk at the Scotland Office in Edinburgh, Mr Moore appears not to be suffering from constitution fatigue - yet. He suggests that, after months of inactivity, the game is afoot with a "step change" in the intergovernmental talks.

"None of us is underestimating the challenges we face or the issues for each government and our respective parliaments. We are now in serious discussions, which we weren't just a few short weeks ago, and we are doing them in good faith and very professionally and I look forward to doing more of them on Monday."

In an exclusive interview with The Herald ahead of the Lib Dem annual conference, which begins on Saturday, the Secretary of State makes clear the two governments have stopped the political posturing and expressed their views on all the key points with "full candour".

Asked if he could, therefore, be candid and say where Mr Salmond and Ms Sturgeon stood on one ballot question or two, he replies: "No. All the issues are on the table, all the details are being worked through. Nothing is agreed until we see what may look like a full package. That's not yet where we're at but we are motoring ahead on all the key issues."

Asked if he could say for sure there will be an agreement, Mr Moore replies: "No, that's the nature of all discussions like this, isn't it? Until you go through all these discussions and negotiations, you have to consider all possibilities as an outcome but we would not be entertaining this or spending so much ministerial time and senior civil service time on this, if we did not think it was possible or there was a good chance to do it."

Mr Cameron had previously expressed the desire to shake Mr Salmond's hand on a deal by the end of this month but that now, it seems, has been pushed back a month. October 22 appears to be the deadline to allow the Scottish Government to meet its timetable for introducing a referendum bill at Holyrood early in 2013. The Conservative conference ends on October 10 and the SNP one begins on October 18; the likelihood is that any meeting between the PM and FM will take place between these dates.

"Where the PM and FM get slotted into this is obviously a matter about whether or not there has been an agreement or there is the prospect of an agreement later on. But we are all working conscious of the need, irrespective of who does what when, to have an arrangement in place by late October, which then helps the Scottish Government to deliver their timetable of introducing the Bill in the spring."

It is possible Mr Moore and Ms Sturgeon will have several more meetings to sort out the technical details before their leaders seal the deal, probably in Edinburgh.

The Scottish Secretary makes clear he is not contemplating failure.

"We're working through the detail. We won't know until we've worked through all that whether an agreement is possible and, if so, how. It's not very informative or helpful to say we are this close to it or otherwise. We are just working through all the issues. It's one of those classics - nothing is agreed until everything is agreed," he adds.