The scene is The Blue Boar in the High Street, Lulverton. The occasion: the stag party planned to celebrate Sergeant Bert Martin''s retirement after thirty years'' service. But Bert had still until midnight before Bradfield was due to step into his shoes. At nine twenty-five Jimmy Hooker was still very much alive, if a little the worse for wear, when he barged in on the party in the upstairs room. At closing time he was dead in the saloon. ''And I don''t think, '' said ''Pop'' Collins, licensee of the Blue Boar, ''that it was in the way of nature.