During a TV news report, the presenter says she spoke to "Commander Robert Dixon," and Dixon appears on the screen with the caption "Inspector Robert Dixon". Commander is a higher rank than Inspector. At the time Dixon was an Inspector, though he was promoted later on.
When Gareth Pierce approaches Dixon on the steps outside a building, the rank insignia on his epaulets denotes a Commander in the Metropolitan Police or Assistant Chief Constable in one of the other territorial police forces, and the oak leaf braiding on the peak of his cap runs along the inside and curved outside edges. Commanders and Assistant Chief Constables have only have single braiding on their caps' outer edge.
In court, Inspector Dixon states that he "never even spoke to Gerry Conlon". Some minutes later when Gerry Conlon is being interrogated, the accused says "You told Inspector Dixon that you had committed a robbery" holding Gerry's statement. There's proof that Inspector Dixon and Gerry spoke, thus making his testimony in court false. Any defendant attorney would have noticed this.