One by one, the citadels of power in Britain have been assailed. First there was the financial crisis. Giant institutions of the City of London, previously presumed to be immovable features of the economic landscape, teetered on the brink of collapse. Then there came the scandal of MPs' expenses. One of the world's venerated legislative bodies was found to host endemic theft from the taxpayer. Now there is phone-hacking. This time, it is giant media companies and the police who stand accused of moral dereliction - the former by tolerating widespread criminality in the methods it used to get stories, the latter by failing to investigate those methods.

The problem, as neither side can easily admit, is that police and tabloid journalists have been in collusion for many years. Cosiness when it comes to sharing information is one thing, but it now seems undeniable that money was routinely changing hands. That is corruption, plain and simple. The public, reasonably preferring the police to enforce the law rather than pervert it, is appalled. At the same time, the government has been flat-footed in its response: slow to set up an inquiry with robust powers to compel witnesses to answer questions under oath, slow to express resistance to News International's bid to take 100 per cent ownership of BSkyB.

There is a simple explanation for that queasiness. The same vested interest that stopped police from investigating shabby practices at News International prevented David Cameron from interrogating his own relationship with that organisation. For as long as News International delivered political protection - or a credible threat of political annihilation - it enjoyed a degree of immunity. Thankfully, that immunity was not universally recognised. The Guardian and a pair of Labour MPs, Tom Watson and Chris Bryant, deserve credit for tenaciously pursuing phone-hacking.

Ed Miliband deserves recognition for taking a prompt and irreversible decision to set himself against Rupert Murdoch's media empire. The House of Commons has generally made some progress in restoring its authority as the seat of national sovereignty in opposition to an unaccountable global media giant. It asserted with near unanimity the only possible view of News International's offer to buy BSkyB given recent events: that the bid should be abandoned. No wonder Mr Murdoch withdrew the offer.

But expressions of outrage do not themselves clean up British public life. Phone-hacking is the third chapter in the trilogy of scandals. The precedents of the previous two are hardly encouraging. Bankers were bailed out and promptly cobbled together a semblance of financial business as usual. The regime of MPs' expenses has been reformed, but parliamentary business remains as opaque and unfathomable to ordinary voters as ever. There is little sign that the public has more faith in its MPs now than two years ago. In both cases the initial spasm of outrage was never effectively translated into structural reform. With MPs' expenses, there has been meagre recompense in greater accountability and visible justice. In the case of bankers, there has been none.

David Cameron came to office vowing to "disinfect" public life with a dose of bright sunlight. The phone-hacking scandal has given the Prime Minister the ideal opportunity to honour that pledge. It is not encouraging that his instinctive response was to lurk in the shadows, waiting to see which way the wind was blowing.