Actually Ai Weiwei was arrested on his way to Hong Kong and hasn't been seen since.
The above plea signed by various British arts representatives refers to a previous report in The Guardian:
"Chinese police are investigating 
outspoken artist Ai Weiwei  for "suspected economic crimes", the state news agency, Xinhua, has  announced. Authorities had not previously acknowledged police action  against the 53-year-old, who went missing on Sunday after being stopped  by officials at Beijing airport.
The single-sentence report,  deleted shortly after it appeared, did not explicitly refer to his  detention, and there was no word on his friend Wen Tao, 38, who has also  been unreachable since his reported detention on the same day.
"Earlier,  outgoing US ambassador Jon Huntsman had raised the artist's case in a  strongly worded speech in Shanghai, describing him as one of the  activists who "challenge the Chinese government to serve the public in  all cases and at all times".
"The artist's detention has 
sparked an international outcry,  with the US, Britain and the European Union criticising a crackdown on  dissidents and activists. Germany's foreign minister, Guido Westerwelle,  has summoned the Chinese ambassador over the issue.
"Police  visited Ai's studio three times in the week before he went missing. "He  felt a premonition that he would be detained," his wife Lu Qing told AP.  "He told me something might happen to him."
She said she was particularly worried about his health as he takes medication for several illnesses.
While  Ai had repeatedly clashed with authorities, friends had already warned  that this case appeared more serious because police had removed dozens  of computers and documents from his studio and had questioned his  assistants.
"Earlier in the day, Ai's mother, Gao Ying, told  Reuters: "I think they detained him for a reason. If they think they  have something, it's certainly a fixed case, an injustice. I think  they'll concoct some things against him."
"Gao said she had been  "filled with a bit of dread" since Ai angered authorities by listing the  names of children who died when schools collapsed in the 2008 Sichuan  earthquake. Officials censored discussion of the subject after public  anger developed over shoddy construction.
"On the Sina microblog, 
China's  domestic equivalent of Twitter, censors deleted many messages about Ai,  and a search for his name produced a warning that results were not  shown due to local regulations. But internet users fought back with  typical ingenuity. Several used the words "ai weilai" or "love the  future" – which looks and sounds similar to his name – to call for his  return. One wrote: "I really don't dare believe that in this society,  even love for the future can disappear." "