Jeremy Corbyn doesn’t do prime minister’s questions very well, so some will tell you. Whatever you think of this claim, it was clear today that the Labour leader is finding his own tone on the human cost of austerity while Theresa May is struggling at the weekly lunchtime duel. Today she set herself up as tough in contrast to Corbyn’s compassion but toiled through the session with her clumsy blows overshadowed by one of her own mistakes.

The Labour leader’s questions on cracks in the social security system and the punitive impact of benefit sanctions showed him on more comfortable ground. He doesn’t do gladiatorial and his allies know this. What he does do is try to probe the prime minister on the subjects that mean most to his political project, like spending cuts and Tory attempts to roll back the frontier of the welfare state.

Corbyn ranged across the benefits system and highlighted the case of David Clapson, 59, a diabetic former soldier who died after he was stripped of his jobseeker’s allowance under coalition reforms because he missed an appointment with a job advisor. Corbyn is right to pick out such stories because they provide a stark reminder of the individual impact of government policies. He built up to a strong attack on the “institutionalised barbarity” of benefit cuts which will ring true to many of his MPs who hear these cruel cases in their constituency offices.

May’s comeback was utterly predictable – accusing Labour of profligacy – but underlined the political challenge facing Corbyn. He feels duty-bound to highlight callous Conservative changes but served up a question which May could easily knock away. Labour wants “no assessments, no sanctions and unlimited welfare”, she cried, in a line straight out of the David Cameron playbook.

This, one fears, was her clip for the evening news bulletins. Corbyn is right to ask questions about what has gone wrong with the welfare state but the public has voted in the Tories for two elections in a row even though they have made no attempt to hide their zeal for slashing social security.

May also had another line for the six o’clock shows but it wasn’t one she had intended. She got herself in a complete mess when Corbyn praised former whip Conor McGinn on the birth of his daughter – delivered by the St Helens North MP on his living room floor – and ended up congratulating the Labour leader on the arrival of a non-existent grandchild.

The PM was evidently embarrassed while Labour MPs revelled in the mistake with the chuckling and rolling around on the green benches that politicians like to do when they hear something funny.

It underlined what was a faltering performance from May, who blamed former Tory chief whip Patrick McLoughlin for her confusion over the baby. May has tried to add a few rhetorical flourishes to the steely delivery she honed over six years avoiding the pitfalls of the Home Office – but looks lost when she departs from her script. Her ability to deliver a joke – which we saw briefly when she first took over in July – has declined from nominal to non-existent.

Corbyn, however, realises that joshing is rarely for him and he sounded stronger when concentrating on substantial issues. Little can be more sobering than a bereavement and he called on May to spend £10m to fund the cost of child funerals. Corbyn cited the terrible situation which faced Carolyn Harris, Labour MP for Swansea East, who was forced to take out a bank loan to bury her son when he died in a road accident in 1989, yet May could only point to the social fund already available. It was a weak answer.

It seems trivial to pick out political winners and losers when such serious issues are being debated but it is clear that Corbyn is improving at his duels with May. These clashes lack the bitter edge seen when Cameron the showman took on Ed Miliband, who liked to provoke and poke fun at the former PM, but MPs still look for a morale boost from a decent performance. They got it today from the Labour leader who can learn from May’s mistakes. Corbyn struggles with the sardonic style of PMQs but substance is helping him each week.