In the novel The Seven Dials Mystery, Inspector Raglan is a police officer (presumably from the local force) who investigated the death of Gerald Wade during a house party at Chimneys. Lord Caterham grumbled to "Bundle" about Raglan because of the fuss he made. Apparently he tested "everything imaginable for finger-prints" when like many others, Lord Caterham thought the obvious explanation was an accident. Lord Caterham thought Raglan "never got over that business four years ago. He thinks every death that takes place here must necessarily be a case of foul play fraught with grave political significance." Lord Caterham is refering to events in The Secret of Chimneys which took place four years earlier. But this is a continuity error because Raglan doesn't actually appear in The Secret of Chimneys!
In The Murder of Roger Ackroyd, there is an Inspector Raglan who appears together with Colonel Melrose, the Chief Constable. Since Colonel Melrose also shows up as Chief Constable in Seven Dials, it is a fair assumption that the pair here are the same as that in Roger Ackroyd. So Raglan, like Melrose, are cross-over characters who link the Poirot and Battle universes.