Focus Zetec 2002 tdci Immobiliser problem

Hi, I have had trouble starting my focus since last night, basically
it won't start . . . you can hear it trying too but it doesn't . . .
battery and all is fine . . . I wait for the usual lights to go out
and then nothing . . . I have heard that this could be the
immobiliser, how do you fix this? Is there a method to do it
yourself, rather than forking out money I can not afford to get
someone out

thanks
Peter