The pilot went through 30 rewrites and four separate story changes before the BBC were satisfied with the plot . It would be rewritten again following the September 11 attacks , with the purpose of acknowledging the series premise as battling terrorism . Filming took place mostly in London , England using a long lens camera . The idea of using split screen , regularly used by the series , came from Nalluri after filming wrapped up for the episode , and was not inspired by the American television series '' 24 ''
September 11 attacks
September 11 attacks
Thou Shalt Not Kill (Spooks)