Conservative MPs are becoming more publicly impatient with their partners in the Coalition. During yesterday's Prime Minister's Questions, two backbenchers berated David Cameron over the influence of Nick Clegg and his colleagues in relation to public service reform. When private frustration erupts in the most public of political forums there is usually trouble ahead.

How quickly the circumstances of an election result are forgotten. Tony Blair used to complain that within weeks of winning landslides he was being attacked by Labour MPs. In this case, Conservative MPs, impatient ministers and advisers would probably not be still enjoying a whiff of power if it were not for the Coalition and its increasingly complex dynamics. They did not win on their own.

Coalitions place unavoidable constraints on the reforming ambitions of a single party. So much is obvious. What is much less obvious is how the necessary limits of the reformers' vision also place formidable obstacles in front of them. These limits relate to the role and responsibilities of government and have already been tested in relation to Andrew Lansley's NHS reforms. Lansley had wanted to hand over formal responsibility for the delivery of healthcare, but has found it impossible to do so. In a less politically explosive context, the limits are surfacing again with Michael Gove's attempt to implement so-called free schools.

The schools are not "free" because they cannot be if a government has a sense of society, as this one claims to do. The activities of one school are bound to impact on another and to some extent on the wider community too. Not surprisingly therefore, free schools are accountable to the centre. Here are some of the constraints outlined on the Department of Education website.

"Groups running free schools cannot make a profit. They will be subject to the same Ofsted inspections as all state schools and will be expected to maintain the same rigorous standards. The admissions arrangements must be fair and transparent. Free schools are expected to be open to pupils of all abilities from the area and cannot be academically selective."

Of course more radical Tories, including almost certainly Gove himself, would like to allow companies that run free schools to make a profit. I am told that would be Cameron's preference too, if he had the political leeway. No doubt they would also overtly support selection in some form or other. But the Liberal Democrats cannot be blamed or praised for these additional constraints. The Conservatives pledged both before the election. Of course there will be more subtle selection. Prominent middle-class parents, like the ubiquitous writer Toby Young, are not setting up a free school in the hope that their children will join a bunch of pupils expelled from other nearby schools. They want the equivalent of a private school intake without paying for it.

In theory, they are not free to secure such an option. Even a government that believes there is such a thing as society, but it is not the same as the state, is compelled to declare explicitly the state will not let the free school be free to do what it wants.

This week, Clegg set out some additional constraints. They include a role for local authorities. His words were reported as being in conflict with Gove, but I understand the Education Secretary was annoyed by the implication of partisan distinctiveness rather than most of the substance. Gove, too, recognises that councils have a role. Clegg was quite emphatic giving one example of how councils could be more effective "by setting higher standards for all schools in their area".

This seemingly straightforward assertion raises many questions. Who decides what are high standards? What does Clegg mean by "setting" high standards? Could a council intervene if a free school fails to meet its definition and in what form?

Clegg finished with a Blair-like flourish declaring: "Free schools, yes, but only if they are fair schools too." In a way which the former Labour leader turned into an art form, Clegg made what seems like a welcome clarification, but on closer examination raises more questions. There is a famous tension between "fairness" and "freedom", one that fuelled the significant policy differences between Blair and Brown. The then-Chancellor worried partly that Blair's hunger to set "free" public services would lead to unfair consequences.

In this case it is not clear how a free school can be forced to act fairly and still be free. Clegg will have reassured many in his party and beyond by highlighting his commitment to poorer pupils and by declaring that the next group of free schools must be in poorer areas, but part of the assurance arises from the implication that the centre pulls strings. Again, it is too easy for Tory MPs to curse Lib Dems. A commitment to poorer pupils is a long-standing preoccupation of Clegg and his party, but there is no dramatic conflict with the public statements of the Conservative leadership that also placed a pre-election emphasis on poorer pupils.

This is the familiar pattern of "reform". Much is made of "freedom" and "choice", only for various necessary barriers to be put in place to protect standards, fairness and value for money from governments responsible for raising cash and, ultimately, for the quality of public services.

Blair blamed Brown when the barriers surfaced, but towards the end of his leadership Brown gave way to Blair's ambitions and still the conflicting objectives were there. Now some Conservative MPs attack the Liberal Democrats, but if they had won a landslide they would face the contradiction between wishing to encourage innovation and efficiency from local providers and being responsible nationally for public spending and delivery.

There is no solution to the conundrum, so perhaps the focus of reform will move on to getting the best teachers to the poorest areas, paying them much higher salaries for the challenge, sorting out those excluded from schools, ensuring that pupils who leave at 16 get the chance to train. In fairness to the previous government and the Coalition, all these difficult areas were or are being partly addressed, yet in political energy and ministerial enthusiasm they are overwhelmed by reforms that make freedom and choice the defining themes when there are unavoidable limits to both.

s.richards@independent.co.uk