Russia and the UK have put on a show of unity during the first visit to Moscow by a British foreign secretary in five years.David Miliband is seeking closer cooperation on issues like Iran’s nuclear programme. But his Russian counterpart is refusing to give way on a separate matter that has damaged relations. Moscow is still rejecting British requests to extradite the chief suspect in the murder of Russian dissident Alexander Litvinenko. Foreign Minister Sergei Lavrov said such a move would go against the constitution. But he said Russia was ready to launch its own inquiry. “If the British provide all the necessary documents, then our prosecutors can carry out the court investigation in the Russian Federation with the help of the British,” he said. Litvinenko was poisoned in 2006. His associates blamed Russian security services. The ex-KGB agent had been drinking tea with two former contacts. His death sparked a rift between the two countries; there followed a series of tit-for-tat diplomatic expulsions. Britain says detailed information has been sent to the Russian authorities. Foreign Secretary David Miliband said the two countries’ differences would not prevent co-operation. “We say that there are other areas where there is more common ground and where we are determined to work together. And I think that is, as I say, a measure of the way in which this relationship has developed – that it is principled, it is clear, it is hard-headed on both sides,” he said. Tensions over other matters such as claims of spying have also cast a shadow over business and trade.