A...for ASSASSIN

Billionaire Sir John Prescott was found one night in his Leeds villa with his throat cut. The murder weapon was strange knife whose wooden handle bore a crudely engraved "A". The handle also contains the fingerprints of all the family members but everyone has a good alibi.
The inheritance at stake is enormous. However Sir John's last will and testament requires the seven possible heirs to appear a month later to collect the fortune, but should there be more than three no one will receive a thing. A deadly game ensues: who will win?