Ukip’s joy was unconfined on Friday morning when Nigel Farage's army of insurgents capped an extraordinary six months by winning a second seat in the House of Commons.

But if the party is set to enjoy a “bounce” in the polls as a result of the Rochester and Strood by-election victory, it isn’t evident yet.

And there are good reasons to doubt Farage’s bold boast that, because of the Rochester result, the result of the May 2015 general election is “unpredictable beyond comprehension”.

The latest polling by YouGov for the Sunday Times shows little change in national voting intentions with six months to go. It has Labour and the Tories tied on 33 per cent with Ukip on 16 per cent and the Lib Dems on seven per cent.

A week ago, the same pollster had Labour ahead on 33, with the Tories on 31 and Ukip on 18. In other words, allowing for the margin of error, there’s no significant change – indeed Ukip have actually dropped a point.

In reality, Mark Reckless's margin of victory in Rochester was well short of the benchmark set by Lord Ashcroft's pre-election constituency poll. That had pointed to a Ukip lead over the Tories of 12 percentage points. In the event, Reckless beat the Tory candidate Kelly Tolhurst by only seven points.

Crucially, Ashcroft’s pre-election polling also suggested that the Tories might well win the seat back in the general election: many of the electorate were happy to lodge a protest vote now, but would return to voting Tory when it mattered.

YouGov’s findings - and Ukip’s below-par result in Rochester – give little encouragement to any Tory (or Labour) backbencher who might have been thinking of following Douglas Carswell and Mark Reckless and defecting to Ukip.

In other words, Farage’s tough message for Tories defending small majorities – “join Ukip or lose your job” - sounds a little desperate: without Tory defections, Farage could struggle to maintain his party's momentum.

And Ukip’s famous “momentum” is questionable. David Herdson of Political Betting comments: "Their [Ukip’s] polling, in the low- to mid-teens, is only marginally up on 12 months ago and is of a level that would not return a significant number of seats at a general election given their vote distribution.”

Herdson argues that Ukip has a problem because it is “riding two horses in opposite directions... On the one hand, those politicians most likely to defect are still Conservatives. On the other, Ukip is increasingly chasing the Labour voter, perceiving – probably rightly – that there are now more soft votes in the red column than the blue one.

“However, the net result of that contradiction is the sort of awkward and unconvincing speech Mark Reckless gave after his win where he tried to proclaim himself the voice of White Van Man. To nail that strategy, what Ukip really needs is a Labour MP to defect. I’m not holding my breath.”

Sky News revealed back in August that Ukip were planning to target heavily 12 seats in the general election: nine held by the Tories (Boston and Skegness, North Thanet, Forest of Dean, Sittingbourne and Sheppey, Aylesbury, East Worthing and Shoreham, Great Yarmouth, Thurrock, South Thanet), two by the Lib Dems (Eastleigh and Portsmouth South) and just one Labour seat, Grimsby. (The latter is a special case – as the Mole reported on Friday, Grimsby’s fishermen loathe the EU for its fishing quotas.)

Without further defections, and without a second surge in the party’s polling - enough to take Ukip’s national share from the teens up to 24 per cent or so - those 12 seats plus Clacton and Rochester and Strood still represent Farage’s best chance.

The person far more likely to be in the position of kingmaker if we’re left with a hung parliament next May is Nicola Sturgeon, the newly elected leader of the Scottish Nationalists.

A series of polls has shown a dramatic SNP surge in the wake of their defeat in the independence referendum, suggesting that traditional Scottish Labour supporters are punishing the party for having campaigned against devolution hand-in-hand with the Tories.

The most recent poll by Survation for Scotland’s Daily Record puts the SNP on 46 per cent, Labour on 24, the Conservatives on 17 and the Lib Dems on six.

As a result, Electoral Calculus projects an SNP landslide, leaving Labour with as few as four of the 41 seats it currently holds in Scotland.

Even if the Tories and Ukip were prepared to form a coalition or Commons pact of some kind, the numbers would not add up: the only combination that could produce the 326 seats needed for a Commons majority would be an alliance of Labour and the SNP.

In her inaugural speech as SNP leader, Nicola Sturgeon was optimistic that her party would hold the balance of power and that Labour would pay a price, being forced to "deliver real powers for our parliament" as well as rethinking the "endless austerity that impoverishes our children" and thinking again about "putting a new generation of Trident nuclear weapons on the River Clyde".

Then came the music for Ed Miliband’s ears: "My pledge to Scotland today is simple - the SNP will never, ever, put the Tories into government.”

Therein lies the difference between the SNP threat to Labour and the Ukip threat to the Tories.

Miliband could lose ten, 20 or even 30 seats to the SNP north of the border and still become prime minister.

Cameron risks losing votes to Ukip which will – as he keeps saying – only bolster Ed Miliband’s chances of getting to Downing Street.