Harriet Harman’s failure to reconcile her public and private faces has had such a cataclysmic effect on the credibility of the Labour party this week that it becomes clearer by the minute that she has demonstrated not so much hypocrisy as the more fragile spots in party policy and unity on education. The hysteria has been more intense because she has transgressed the spirit, rather than the letter, of Labour policy.

At last October’s party conference, Ms Harman sat - her own lips pursed - as education spokesman David Blunkett routed a spirited policy attack from Roy Hattersley with the promise: “Read my lips, no selection.” It must have been about then that her son sat his entrance tests for St Olave’s grammar school. But David Blunkett must also have known that his conference victory concealed uneasy compromises.

He published two constructive policy documents last year, one promising to restore equity to an increasingly elitist school system, and the other committing a Labour Government to the pursuit of high standards for the many, rather than the few. But there was no commitment to abolish existing grammar schools; only a suggestion that local parents should be balloted on their future. Nor were grant-maintained schools to be wiped out, but turned into foundation schools, with the same funding and admission arrangements as all local neighbours. Parent choice was to stay.

Agreement on all these issues came tough for Labour, driven through by leader Tony Blair’s well-founded conviction that middle-class voters like him (and Harman) wanted such schools brought into the local system on equitable terms, rather than driven out. But the rub comes on the last two points: admissions systems and parent choice, and their compatibility. So far, the policy line on admissions procedures - local authority approval and appeals tribunals - is dangerously vague, and it must be doubtful whether any system can perfectly reconcile choice with the balanced intake that every comprehensive needs.

So Harriet Harman can argue with justification that as a parent she must operate in the system as it now is, rather than the way she would like it to be, as a politician. As a parent she is operating in the way that London parents in particular have always done, looking beyond borough and inner London boundaries in search of a better choice.

The Dromey-Harman household is in Southwark, a potent example of educational divisiveness and obvious target for “Excellence for Everyone” reforms. Nearly a third of the borough’s children are creamed off to independent schools; there is a city technology college and four Roman Catholic grant-maintained schools with intakes which may be balanced on ability, though not disadvantage. The remaining comprehensives are in effect secondary moderns. Southwark tried to use a banding entry system but abandoned it in l992 as unworkable because there were so few band one children.

Though its schools have remained near the bottom of the league tables, Southwark has been working hard on improvement and rose several places last year. St Saviour’s girls’ school leaped up the tables after the arrival of a new head and applications have tripled in consequence, but there is still a long way to go for other neighbourhood schools. We understand enough about school improvement by now to know that regeneration requires a long and concerted effort by head, teachers, LEA, governors and parents, and Southwark schools do not yet have all these ingredients in place.

The nub of the question for the Labour party is what part Harriet Harman and her husband Jack Dromey could and should play. They have acted like any other parents who know how to exercise choice and are not committed to the comprehensive system as it now is. But as Labour leaders more is demanded. Other Labour politicians at local and national level have unhesitatingly used the comprehensives they believe in, taking a lead (as community leaders should) to make changes where necessary and beef up the intake. A demanding stance, but where a Blair or Harman goes, runs the argument, other middle-class families will follow.

There is a middle way, of course, a fudge which would not have torn the party apart. There are good, genuine comprehensives to be found in London and the family could have moved into a suitable catchment area not too far away in good time. In the end, it is picking a grammar school which has proved most unforgivable for colleagues and critics.

However her decision is rationalised, its results have brought Harriet Harman’s own career and the fortunes of her party to the crossroads. She has done nothing illegal or immoral, yet her political future is in jeopardy because her credibility as a front-bench spokesman has gone. It is not yet clear whether the public is more revolted by symptoms of hypocrisy or the appalling baying of the hounds on Left and Right. But it is certain that the Labour Party collapsed in crucial Commons debates on both education and health when it should have been the failures of Government policies which were exposed. Labour’s educational policy was looking good, but it is clearly not robust enough when such a choice can threaten to bring down an Opposition.