Stay tuned for coverage from 12:00
12:03: Cameron pays tribute to the 3 soldiers killed in Afghanistan this week.
12:04: Tory backbencher Phillip Davies opens proceedings by calling for prisoners to be denied access to Sky TV. Cameron's response is true blue: too many prisoners, too many of them in Britain illegally and not enough money.
12:05: Harman rises, talking on unemployment, which has risen this morning. It's effectively cuts now versus cuts later. Cameron regrets the situation (to much noise from the opposition) and pledges that back to work initiatives will be enacted in next week's budget.
12:08: Harman re-iterates her point. Cameron responds by saying that Labour hasn't given any concrete spending plans of her. Evenly matched so far and not very revealing.
12:09: Now Harman uses the OBR's findings that the economy is not as bad as expected. Cameron dodges the question by appealing to his government's transparency, which doesn't answer the question. But Cameron responds with the other side of the story: growth is slower than Darling forecast.
12:11: Harman says the Tories are talking down the economy to facilitate cuts at the expense of business confidence. That's a powerful argument and Cameron responds by pointing out that Labour has got us into the mess.
12:12: Now Harman strikes at the 'blame Labour' line that Cameron takes - if Labour's spending plans were so bad, why did Cameron follow them until the end of 2008? 'It's a magic roundabout with him,' she quips ineffectively. Cameron calls once more for Labour to reveal where it will cut.
Labour may have been inept, but the Tories backed them most of the way; I think Harman edged that exchange.
12:15: In response to a question from Margaret Hodge, Cameron suggests that he may bring forward the Cancer Drugs Fund.
12:17: Labour MP John Woodcock wants the Astute Class submarines, which are being built and maintained in his constituency, will be preserved from the Defence Review. Cameron says they won't because, like Labour when in government, everything has to be reviewed, no matter how important to a region and the defence of the country.
12:19: The second question of the day of RDAs - Cameron says that those which offer essential services will be protected and those that serve a political purpose (2/3rds are found in Labour marginals) will go.
12:21: Douglas Carswell wonders why there is a referednum on AV, which did not feature in either coalition's manifesto, when there is not a referednum on all matters EU. Cameron says that there will be a lock-down on further EU treaties - well deflected.
12:23: Sir Alan Beith asks about protecting upland farming by extending affordable housing and broadband to ensure that the young remain. Cameron agrees, but target driven housing schemes are failing communities.
12:27: Urgh... A Tory plant: how much is the debt for each household? £22,000.
12:29: Graham Brady wants protection for savers in the Budget. Cameron says that keeping inflation low will be essential. Nothing on CGT, but is this the new accord between Cameron and the Tory backbenchers?
12:32: Great moment. Tory Harriett Baldwin has a plant about ring-fenced health spending to jeers from Labour. Bercow intervenes with a twinkle sayong: 'A government backbencher supporting the government is not that extraordinary'. Cameron repsonds 'Indeed Mr Speaker, we all remember you doing that.'
VERDICT: I seem to be a minority of one, but Harman edged the main battle. The exchanges were heated, and her strategy was well rehearsed and I think we've seen the weakness in Cameron's blame Labour strategy: it doesn't bear repetition. Cameron was far better against hostile backbenchers, particularly from Tories Davies and Carswell. The good natured quip at Bercow's expense showed his brilliance at the dispatch box. The advantage about the new format is that you tend to forget the leader of the opposition's questions after the backbench deluge.