A few hours before Andy Burnham gave his inaugural speech in the House of Commons as UK prime minister, his predecessor Keir Starmer, whom he so brutally defenestrated over the summer, intervened to give him a parting headache.

Starmer’s announcement that he was stepping down as an MP – one final U-turn from the particularly pivot-prone former prime minister – will spark a byelection that will be an early test of his successor’s brand of self-proclaimed progressive politics.

His party will expect to retain Starmer’s old Holborn and St Pancras seat in affluent north London. But the vote will be useful as a measure of the strength of Burnham-era Labour against the threat from the left: a one-on-one with the Green Party. The Greens will throw everything at it, including, possibly, their leader Zack Polanski.

Burnham came to office via the pie-worshipping former pit towns of Makerfield while proclaiming he had a recipe to defeat Nigel Farage’s right wingers in Reform UK. Now he must quickly reposition Labour for a battle to defend its other flank.

Starmer’s awkward little goodbye present shortly before Burnham’s speech did not stop the new prime minister from showering the vanquished man with effusive praise.

Starmer, he told the Commons, had given Britain a “lifetime of service” and his landslide election win and first two years of government had “created a platform” for Burnham’s new-look administration to forge ahead with a revamped agenda.

“We will use it well,” he said, continuing his generous remarks towards his predecessor.

That sense of generosity clearly did not extend towards the legion of loyal Starmerites whom he sacked the moment he got the keys to Downing Street in July. Burnham’s new-look front bench line-up suggested as much. Gone were most of the Starmer-era heavyweights such as Rachel Reeves, David Lammy and Steve Reed.

Instead, Burnham was flanked on his front bench by several of the so-called “Northern queens” who helped to mastermind his stunning ascent over two months from the mayoralty of Greater Manchester to leader of a nuclear-armed G7 nation.

Labour’s deputy leader, Lucy Powell, sacked by Starmer but now restored to cabinet as education secretary, sat two seats to Burnham’s left, scrawling notes for him as others spoke. Two to his right was Anneliese Midgley, who was largely ignored by Starmer but is now one of Burnham’s most important parliamentary enforcers as chief whip.

The queen of queens, however, sat closest to Burnham at his right-hand side. Sheffield Heeley MP Louise Haigh entered the chamber and took her seat with the prime minister at 3.37pm to loud cheers from Labour’s backbenches for the new regime.

Seen from the vantage point of the press gallery, Haigh appeared to be the one who Burnham consulted most as he scribbled retorts during the question-and-answer session. She discreetly whispered in his ear each time someone else rose to speak in the chamber.

Barely 22 months ago, Starmer effectively sacked Haigh as transport secretary in a brutal move that seemed to have ended her career. Now her old boss was heading for political retirement largely enforced by Haigh’s plotting while she, as Britain’s powerful new first secretary of state, runs the country whenever her new boss is away.

When Westminster’s pendulum swings, it can truly clatter those caught unprepared.

Burnham’s speech was largely a repeat of his promise to remake British politics after 40 years of post-Thatcher neoliberal endeavour. He said his government would be a “circuit breaker”. He said the state would take the wheel in the drive to generate growth. He explicitly blamed Brexit for much of Britain’s economic woes.

Burnham also suggested that where he agreed with Starmer’s approach, he might leave his policies in place. “[But] where necessary, I will not hesitate to make changes.”

At this point in his speech, Burnham did something that appeared a little unusual. He took off his trademark glasses to read. Isn’t it normally done the other way round? The essence of this new prime minister’s approach is that he will do things his own way.

There was low-level noise in the chamber throughout Burnham’s speech. Tory MPs chattered incessantly as he spoke, pausing only to try to put him off with their favourite one-word heckle: “tax.” The main opposition party’s mantra is that Burnham will tax anything that moves to pay for his long list of promises.

The Tories quietened down when it was the turn of their leader, Kemi Badenoch, who gave the first reply to her opposite number’s speech in the luxury of a quiet chamber.

She sarcastically thanked Burnham for “finally finding the time to come to parliament” to lay out his plans. The eyebrows of most people present furrowed in confusion: this was the first day back after recess. He could hardly have done it any sooner.

Badenoch said the prime minister’s role was “a very different job from being mayor of Manchester,” while she also acknowledged that it was “not as easy as it looks”.

Starmer’s stilted approach sometimes made it look like the most difficult job in the world. Burnham’s more relaxed, assured manner suggests he might make it look a little easier. Britain’s entrenched problems suggest he faces tougher times ahead.