There were many, including myself, who questioned whether Alex Salmond's victory over Alistair Darling in Monday's Scottish independence debate would have any effect on voting intention in the referendum. But the first poll carried out since the contest suggests it may well have done. A Survation survey for the Daily Mail puts the No side down four points to 53 per cent and the Yes side up four points to 47 per cent.
This is, of course, just one poll (more will be required to determine whether the race has narrowed) and, in common with all others since the campaign began, it puts the Yes side behind. But with three weeks to go, the gap is still narrow enough to give Unionists sleepless nights. The nationalists have long counted on a strong finish, as in the 2011 Scottish parliamentary election (when the SNP overturned a double-digit Labour lead to win a majority), and Alex Salmond appears to be delivering.
Along with Douglas Carswell's defection to Ukip today, the uncertainty over the future of the Union, that 307-year-old institution, is a reminder of just how unstable British politics is at the moment.