According to this the information is being gathered by a US firm called Lockhead Martin, on behalf of ONS.
What freaks me out is this:
"Lockheed Martin ? which does 80% of its work for the US defence department ? assists more than two dozen American government agencies and is involved in surveillance and data processing for the CIA and FBI. It has controversially provided private contract interrogators to the Abu Ghraib prison in Iraq and Guantánamo Bay in Cuba. All US-based companies are subject to the Patriot Act, which allows the US government to have access to any data in the company's possession. Campaigners have warned this could give the US government access to detailed and personal data on the UK's entire population"
Anyone heard of this? I'm now less enthusiastic about filling it in.