As many hospitals struggle to cope with a surge of Covid-19 patients, the most important judgement yet to be made about 2020 is how much difference it would have made had England been pre-emptively locked down in September. This is not an academic question. Because there were two separate occasions in September when the prime minister's political and scientific advisers urged him to impose tough national restrictions and suppress the incidence of the virus back to low levels. It is well known that on 21 September the government's Scientific Advisory Group for Emergencies recommended a short "circuit-breaking" lockdown. But I have learned that within Downing Street, it was at the beginning of September that Boris Johnson was urged by officials and colleagues - led by his former adviser Dominic Cummings - to impose tough new controls on our behaviour. I am told that Cummings, his ally Ben Warner and leading members of SAGE were in favour of "whacking it [the virus] early]". According to a source they argued "you should do it now because it will save lives and minimise disruption". But the prime minister and the Chancellor Rishi Sunak argued that "we can't justify it now", so it didn't happen. In early September, at a meeting in the Cabinet Room, Cummings and Warner presented data about how the virus would spread by the end of October without such suppressive measures. They believe they were proved right: "at the end of October, a meeting then replayed exactly what the data team had projected", the source says. It was at the end of October that the circuit breaking lockdown was finally ordered by the prime minister. But by then the virus was already so prevalent in so many parts of the country that the lockdown was the precursor to the widespread imposition of the "stay-at-home" tier 4 in most of England. By chance I interviewed the SAGE member John Edmunds on the 7 September when he warned that the virus was spreading "exponentially". At the time, Cummings was taking advice from Edmunds, among others. Edmunds is one of many scientists who believe it was a serious error not to lock down in September. I asked the prime minister in my interview yesterday whether he made a mistake in rejecting the advice to lock down in September. He said that there were other considerations for him at the time and that there is evidence the tiering system he later introduced was beginning to work, before the new strain of coronavirus became such a pernicious factor. Cummings's record on coronavirus is mixed. His controversial trip to Barnard Castle is widely seen to have undermined public confidence in lockdown measures. But his colleagues - including senior non-political Whitehall officials - say that before both the first and second lockdowns he urged the prime minister to take suppressive measures earlier than the prime minister was prepared to do. A senior official who still works for the prime minister says: "In March Dom was storming around Downing Street shouting 'lock down now'". I am told that an important cause of the breakdown of relations between the prime minister and Cummings in the autumn was that the PM was aware Cummings thought he had "f***ed up" by not locking down in September. It is understood Cummings would give evidence on oath about all this to any future public enquiry.