Labour sources at the Rutherglen and Hamilton West count were either playing a game with us about the scale of their party’s victory or were genuinely surprised.

Early on at the count, they said they thought the swing from the SNP to Labour would be 7% to 8%, suggesting a gain of 15 to 22 seats in Scotland in a general election.

They said a 10% swing, which they claimed they didn’t expect, would suggest Labour gains of 24 seats. Later, they admitted the Labour lead was nudging up towards 10%.

But when the result was declared at around 1.30am, Labour’s majority wasn’t far short of 10,000 and the swing was a massive 20%.

In the trade – the rough old trade of politics – it’s known as expectation management. But I’m prepared to be charitable and suggest that not even the most optimistic Labour official was expecting a 20% swing.

It’s all hypothetical, of course, but the great Michael Thrasher – “the Prof” – suggests that if you take the votes cast for each party in this by-election and apply them to a general election Labour would have 42 seats in Scotland.

Michael Thrasher: SNP loses Rutherglen and Hamilton West seat in ‘spectacular fashion’

Fanciful? Possibly. That would take Labour back to the level it was at in 2010, when under the “great clunking fist”, Gordon Brown, it had 41 MPs in Scotland.

In 2015, when under Nicola Sturgeon the SNP won 56 of Scotland’s seats, Labour held just one, Ian Murray in Edinburgh South. After winning seven in 2017, Labour slumped to one again, the resilient Mr Murray, in 2019.

Does this by-election mean Sir Keir Starmer is on course to win a clear working majority next year. Not necessarily. It’s only a by-election and Labour has been through enough false dawns to fall for that. In 1992, for example.

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No wonder Labour sources at the Rutherglen count were being extremely cautious, managing expectations or were indeed genuinely surprised.

Labour is on the way back, though. In its tartan fightback, this is an excellent start. Potentially, this by-election could be a defining moment in the battle to win the next election and put Sir Keir on the road to Downing Street.

It coincided with a YouGov poll showing Labour’s lead over the Conservatives back up to a hefty 21 points. So much for a conference bounce, prime minister.

The SNP weren’t the only losers in this by-election. If Labour is on the way back and on course to win big numbers of seats in Scotland, the biggest losers after Rutherglen and Hamilton West will be Rishi Sunak and the Conservatives.