Huddled in a damp fire-station a young man connects a laptop, a binocular-style iris scanner and a glowing green machine that records 30 points from a set of fingerprints.
He has already had to damp down a fire started when an unidentified aide told the Daily Telegraph that they understood Britain better than the White House because of a shared "Anglo-Saxon" heritage.
Afterwards Mr Cai and his assistants damp out the fire using long poles tipped with sponges, and then smudge the lines of ash so that the portrait looks like a huge charcoal drawing on the wall of the museum's reception hall.