x5 3.0 diesel sport, automatic, turn over but not starting??

[warmz3]

hi,im having problems with my x5 starting..
95% of the time it starts excellent.. 5% of the time it wont start it will turn over but not fire.. BMW are being useless and arent sure what was wrong, they said nothing came up on fault machine but he found a burnt out fuel pump reley which they replaced and said that will prob sort it, it hasnt. anyone had this problem before??
any ideas what it could be??
let me know asap..
much appreciated..

thanks,
James