The prime minister was asked in Jakarta about the death of Heywood after the detention of Gu Kailai, the wife of the former Chinese leadership contender Bo Xilai, on suspicion of his murder.
Standing next to the Indonesian president, Susilo Bambang Yudhoyono, in the presidential palace in Jakarta, Cameron said: "On the case of Neil Heywood, we did ask the Chinese to hold an investigation and we are pleased that they are now doing that. I stand ready to co-operate in any way that we can. It is very important that we get to the bottom of the truth of what happened in this very disturbing case, this very tragic case."
Downing Street is pleased that the Chinese authorities appear to be taking the death of Heywood seriously. The prime minister's remark that Britain had been asking the Chinese authorities to launch an investigation was designed to answer critics who said Britain had failed to put any pressure on Beijing for fear of offending the growing superpower.
Bo's political death knell has sounded across China as newspaper front pages and hourly news bulletins trumpeted his disgrace and his wife's detention for the suspected murder of Heywood.
The scandal that toppled the high-profile former leadership contender is the biggest political upheaval in China since general secretary Zhao Ziyang was ousted following the Tiananmen democracy protests in 1989. It comes months before a once-a-decade leadership transition.
The official party newspaper, the People's Daily, urged people to rally around the top leadership, saying Bo had damaged the cause and image of both party and state. But while investigations into the couple are continuing, news coverage also sought to draw a line under events, with a Global Times article saying the country had "steadily overcome a bump" ahead of the 18th party congress this autumn, which will unveil the new leadership.
The news agency Xinhua has said Bo is under investigation by central party officials for "serious disciplinary violations" – a statement that has often been followed by corruption charges against leaders in the past.
Bo was dismissed on 15 March as party secretary of the south-western city of Chongqing – where Heywood died last November – but had held on to his politburo and central committee seats, apparently because leaders had not agreed on how to handle him. Many believe that was related to horse-trading before the autumn power transition as well as Bo's powerful connections as the "princeling" son of a renowned Communist party veteran.
The scandal was set in motion in February when Bo's ally and former police chief Wang Lijun fled to the US embassy in Chengdu, where he is believed to have told diplomats he had angered Bo by confronting him with suspicions over Heywood's death. Wang is now under investigation.
But many believe rivals and opponents were already seeking to damage Bo.
Jean-Pierre Cabestan, of Hong Kong University, suggested Wang's flight, while deeply embarrassing to Chinese leaders, was merely "the last straw on the camel's back". Analysts say Bo alienated senior party figures by attempting to leverage popular support to win promotion in the leadership transition.
"He created a coalition of everyone wanting to get rid of him because he became a 'troublemaker' and a factor of division," said Cabestan.
He suggested it was also "a clear victory for reformists" at the top of the party who are pressing for change.
"Behind this case is a power struggle," said Zhang Ming, a liberal scholar at Renmin University in Beijing. "Bo broke the rules and greatly deviated from the [usual] track … This shows that the struggle has intensified."
Wu Qiang, a political scientist at Tsinghua University, said Bo's populism had been a threat: "This is a cleaning-up among princelings," he added.
Xinhua has said Bo's 51-year-old wife, Gu Kailai, and a worker from the couple's home are "highly suspected" of killing Heywood. It added that Gu and the couple's son Bo Guagua had been on good terms with the 41-year-old British businessman but that there had been a conflict over economic interests.
Britain had already asked China to reinvestigate Heywood's death, following suggestions of suspicious circumstances. Family members in China and the UK have dismissed the idea of foul play and said he died of a heart attack.
Heywood's widow was at their home in a blossom-lined, upmarket housing compound in the northern suburbs of Beijing on Tuesday, but did not answer the door. Her late husband's silver Jaguar, bearing a union flag sticker, was parked in the driveway.
"No one is home," said a man in plain clothes who said he was head of security for the compound.
Britain and the US were briefed on the announcements shortly before they were made public.
The foreign secretary, William Hague, has welcomed the reinvestigation, telling reporters: "It's a death that needs to be investigated, on its own terms and on its own merits, without political considerations."
As with many developments in the case, Tuesday night's bombshell announcements were presaged by a bout of speculation about such moves on the country's microblogs.
Despite censorship, many used the services to praise and attack Bo on Wednesday.
"Such a good official. He did so many good things for people … How much do the bad guys hate him!" wrote one internet user.
"He wanted to restore the cultural revolution. He even criticised and fought against his own father [during the cultural revolution]. Where is his goodness? I don't get it!" argued another.
A third noted simply: "What happened in Chongqing told us that no matter what is right or wrong, it is important to stand on the winning side."