Leading article: Debate must be about more than the Speaker

Since the nine-hour arrest of Damian Green and the police searches of his premises, the political fallout has come in distinct stages and moved at what has sometimes seemed a glacial pace. Over the past week, the Metropolitan Police, the Speaker of the Commons and the Home Secretary have all produced separate chronologies and justifications for what happened. They boiled down to variations on "not me, Guv", and "we were only following orders".

Today, MPs – who have been gagging for their say since the affair erupted – will finally be given the debate they want. On the face of it, the motion is simple. It calls for the setting up of a committee to investigate the police raid on Mr Green's Commons office. But nothing about this tangled case has been allowed to be as straightforward as it ought to be. For instance, those MPs who were disturbed by the ease with which police were admitted to an MP's office – and their number goes far beyond Mr Green's party supporters – risk being damned if they support the motion and damned if they don't.

Of course, they want an investigation but the committee being proposed would comprise seven senior MPs appointed by the Speaker in a composition reflecting the composition of the House. In other words, if its deliberations became fraught, the outcome would inevitably become coloured by party politics. The other drawback is that, as with the inquiry into cash for honours, it is proposed that its report be delayed so as not to prejudice the parallel police investigation.

It is no wonder that Conservatives are already busy tabling amendments and that the Liberal Democrats are saying they want no part of it. Both parties are right to smell a rat.

Yet an investigation into what happened, and why, is imperative. Detectives were admitted to an MP's office in the Commons and allowed to search it on what seems, on the available information, the barest of pretexts.

Convention has it that MPs' persons and premises within the precincts of Parliament are privileged. This does not mean that they are above the law but it does mean that a potential conflict is recognised between the need to protect the confidentiality of an MP's dealings with his constituents and the requirements of law enforcement.

It might seem – as it apparently seemed to the Serjeant at Arms, the official who gave her consent for the search – that primacy rests with law-enforcement. But this should never be taken for granted, especially not at a time when police powers vis à vis ordinary citizens have been growing, with full ministerial encouragement. The police must be made to justify their request and obtain a warrant. It would be absurd if it were easier for them to gain access to an MP's Commons office than to search anyone else's home.

The role of the Speaker, Michael Martin, was, and remains, central here. In regretting what happened and undertaking to take such decisions himself in future, "as is my responsibility", he effectively admitted to being in the wrong in this case. He did not, however, tender his resignation, or come close to doing so.

Yet that omission, coupled with hints that he might be aspiring to a third term, created an impression of defiance that was unseemly. With many MPs wanting to call him to account, but without the appetite to depose him, a more elegant solution might have been an early confidence vote. Given that a damaged Speaker is a liability to the Government, as well as to the House as a whole, this might still be the most expedient option for all concerned.