This weekend the PM is at Chequers deciding when to face the voters. Westminster Editor James Cusick reports

Jim Callaghan, Labour's unelected prime minister, loved Chequers. He often told people "It's the most wonderful place to hold a family party." Today another unelected prime minister is at Chequers, but the last thing on Gordon Brown's mind will be entertainment. To go, or not to go, that is the question Brown will ponder before he heads back to Downing Street tomorrow. He will be hoping the decision he makes doesn't repeat the catastrophic error that led Callaghan to delay in 1978 even though he was ahead in the polls; a year later 18 consecutive years of Tory government began.

If the weather is good, Brown might walk in the extensive gardens, look to the Chiltern Hills and decide he may never have a better chance of winning a general election, and securing the right to be Britain's prime minister.

But it's more than likely that Brown will be at his desk, surrounded by computers and looking for inspiration in the documents and emails he has ordered from his closest aides and Cabinet colleagues. While electoral arithmetic points to a "go now" verdict, it will come down to gut instinct and a gamble that if lost, would leave Brown as the shortest serving PM in modern British political history. For a politician who waited a long time for a job he thought was rightly his, the urge to gamble might not be that strong.

The Long Room at Chequers could deliver fighting spirit if he looked in the volumes of Admiral Lord Nelson's diaries on the tall library shelves. But Brown prefers numbers crunched by his senior pollster, Deborah Mattinson.

Mattinson - chief executive of Opinion Leader Research - and her political calculator have been around for 20 years. She has always regarded Brown as a steady, reliable character who was one of the key figures in getting the Labour Party to stop listening to itself and begin listening to other people. Last year when a BBC Newsnight poll put John Reid ahead of Brown as a potential PM, Mattinson told Brown to dismiss it. She has been telling senior Labour advisers for over a year that Brown's lead is "consistent and unchanging". Mattinson's advice will be on Brown's computer screen in Chequers today: it will tell him that seven out of eight polls this month put Labour on at least 39%, four have hit the 40% mark. Today a YouGov poll puts Labour on 43% - 11 points ahead of the Tories. It will tell Brown his lead isn't a volatile projection, and she will tell him, as she always has, to "ignore the headlines" and look at the truth in the numbers. Her final advice? It will say that when Cameron was in the lead, his popularity was transient: she will say Brown's long-term investment as a serious politician is now paying a huge dividend - go for it.

The advice is similar from those in Brown's Cabinet. Most if not all have accepted that if there is some justified hesitancy now, the same hesitancy will still be around in summer next year. It is understood that Brown told some of his key allies to put their analysis down on paper; his desk at Chequers will be littered with advice that points to the same conclusion reached by Mattinson: go now.

Ed Balls, the schools secretary, is perhaps Brown's most trusted colleague. Last week in Bournemouth, Balls almost accidentally described the task facing Brown: he said the prime minister was in the "middle of a crucial week" and there had to be a precise evaluation of "the gamble" that they faced. And like Mattinson, Balls held the firm opinion that the "leadership" of Brown had already been established.

So if there is to be an autumn gamble by Brown, the dilemma for the PM is: when does he go for it?

The last possible election date before British Summer Time ends is October 25. This would mean Brown driving to Buckingham Palace on Tuesday, the day before David Cameron gives his leader's speech at the Conservative conference in Blackpool. The majority of Britain's political media will be in Blackpool - and the immediate conclusion would be that Brown had deliberately wrecked the Tory conference.

Would he care? Around Chequers there are plenty of reminders of another conviction politician and leader, Oliver Cromwell, who put his opinion above that of others. But however much Brown might want to end Cameron's political career, open assassination, as Cromwell found out, might not go down well.

Chequers is the private part of Brown's decision. When he comes back to London tomorrow, the more public part of his deliberations begins. If he has dismissed October 25 as too soon, the next date is November 1. Brown could watch Cameron's speech on Wednesday and decide what, if any, impact the Tory leader had made. Mattinson will have given Brown instant feedback on whether or not the following day's polls will give the Tories any reasons to be optimistic.

Next Monday, Brown is expected to make a major statement in the Commons on Iraq, thought to contain the dominant message that further substantial troop reductions will go ahead: the subtext will be that Brown is ending Britain's part in Blair's war, further evidence that Brown is different from Blair, that his New Labour can be newer still.

If the election were on November 1, October 9, the day after the Iraq statement, would see Brown make the traditional visit to inform the Queen.

But between the tranquillity of Chequers and buzz of a full-blown "war council" in Downing Street next week, what other judgements will be made? One Labour analyst said even the weather forecast for November 1 could be a factor. What if it's very dark, what if there's rain? And after Hallowe'en the night before, will people venture out again to vote?

If Brown is to opt for a snap poll, November 1 looks a serious front-runner, simply because the week before is in the middle of the English and Welsh schools mid-term holiday when some families might be away. And Labour traditionally do better when the poll percentage is high.

Another week would allow Labour to finalise its financing of the election campaign. At Chequers and next week in London, Brown will want Labour's moneymen to reassure him that big spending by the Conservatives won't trample over his poll advantage. On current estimates, it is likely that in key marginals, Tory candidates will be able to spend twice as much as their opponents. But if their message is limited, the size of billboards won't make that much difference.

A November 8 poll, would have to be called by October 16, a month after Bournemouth: will the memory of Brown's triumphant message of change have dimmed in the dark nights of early winter?

Walking alone down the long Tudor corridors of Chequers this weekend, Brown could be forgiven for looking like a man with a problem. He isn't. He is a man who has to make a decision.

Six to 11 points ahead in the polls; the prize of a personal mandate so close; the prospect of winning and then holding the next general election in 2012 bathed in the glory of the London Olympics; the prospect of destroying Tory fortunes for another generation. Three months ago even Brown's most fanatical supporters never thought he'd be in this position.

But the naturally cautious Brown may see the flip side of this potential success. He could be asking himself where the real gamble lies. What if the planned Labour campaign is a dud: he wins, but the early polls prove overly optimistic? How much would this damage Brown and his authority if he became PM, but with a massively reduced majority?

What if the polls have focused too much on the north, and have ignored the south and southeast where key marginals could turn away from a Scotsman with a reputation for being dour? And in Scotland, what if the Holyrood success of the SNP spills over to limit Labour's traditional strength north of the Border?

To go, or not to go, that is still the big question left for Gordon Brown and no-one else.