Line Of Duty bosses are desperate to keep the season finale top secret – in what could be the last ever series.Stars had to sign contracts banning them from saying what happens and will face legal action if they breach it.And all scripts were watermarked so if one gets into the wrong hands, bosses can investigate who it has come from.Martin Compston, who plays DI Steve Arnott, feared this could be the show’s end of duty.He told Saturday’s Jonathan Ross show: "If it ends, well maybe, sometimes, it is best to leave it."Meanwhile, execs have made sure only a handful of people will be able to watch a preview of the finale before it airs on Sunday at 9pm on BBC One.A show insider said: "Security around the episode is tighter than any police.
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