Ai Weiwei: Huawei, Hong Kong and being an artist in exile

China's rise to economic superpower status has not brought with it an opening up of politics or culture. Far from it. The Communist Party has intensified its efforts to suppress dissent of all kinds. Stephen Sackur speaks to China's most internationally-famous artist, Ai Weiwei, who now lives in the UK and not Beijing. He's a refugee and a migrant of sorts, so how has that affected his creative output?