Ten live musicians. One hundred and sixty cameras. Two hundred and ninety-one speakers. Five hundred moving lights. One thousand Industrial Light & Magic visual effects experts, working one billion computing hours. One thousand, six hundred seats. Eight hundred and seventy thousand watts of audio amplification. And 65 million pixels on one state-of-the-art screen. Mark Sutherland tries the first ‘ABBA’ gig in London since 1979 and speaks to the creators.