The Prime Minister wants to see a UK police review in into Madeleine McCann's disappearance because it is "clearly an exceptional case".

That's according to his spokesman, who today rebuffed several potential criticisms of the decision - that David Cameron is bowing to the tabloid press, that it amounts to political interference by the Home Office, and that it will be costly.

Mr Cameron considers it "exceptional" because of the significant public interest, the length of time Madeleine has been missing and the international dimension to the case.

If there are doubters, they may point out that it is perhaps only the first point that is truly unique - there are other children who have been missing a long time, others who are missing abroad.

And has the Home Office interfered by making this "request" to the Met? It is claimed not - because it was not "direction" and the force agreed anyway.

But, as Paul Waugh over at PoliticsHome also points out, this is a particularly sensitive issue in the light of the Government's hopes for directly elected police commissioners.

The Lords voted to change this policy earlier this week (though this will almost certainly be overturned by the Commons...) and one of the main criticisms centres on fears these hugely powerful individuals could make politically-motivated interventions.

In a draft protocol on commissioners, published by the Home Office, it says its "strong commitment to ensuring that the operational independence of Chief Constables will remain".

But when does a "request" become an instruction?

And lastly, the cash. The investigation won't be paid for by the cops, but by the Home Office, thanks to "unallocated" money in the budget which will be given as a "special grant".

But at this time of fiscal austerity, it may be a surprise to some that there is any leftover cash at all. I've asked the Home Office for more details on how much there is and what it is normally spent on - I'll update this post when I get a response.

A quick search suggests in the past this pot of cash has been used to fund an investigation into a fatal fire and policing events such as political party conferences, plus there are reports the Government would help pay for the inquiry into the Cumbria shootings this way.

David Cameron will be more than aware of all these issues and clearly believes it is the right thing to do and that all those involved have acted properly.

His open letter to Kate and Gerry McCann, published in the Sun, indicates he is reacting partly as Prime Minister and partly as a father.

Labour's shadow home secretary, Yvette Cooper, has welcomed the McCanns' request for information and the authorities' decision to do everything in their power. This suggests while there might be questions about how the Government has responded, there's unlikely to be a row.