> The song that Dougan sampled was Chopin's Prelude #4.

It most definatley was not chopin's prelude no 4 or one of his nocturne's
the string section at the start of the piece is elgar's enigma variations movement 1. but rob d didnt sample chopin in the piece. iv searched long and hard to find the original music to the piano solo and i personaly believe that rob d has used the piano solo as an improvisation on the theme of elgar.
At the end of the day i think it is best just to learn it by ear it's not that tricky. i'll give you a head start with the harmony its centered around the key of G minor. three main chords of interest are g minor, d major and f# minor.

I realise this thread is very old but if you e-mail me i can give you my transcribed version of the piano solo in rob d's clubbed to death written in sibelius

hope this helps!