The anus. Slang term used in the Medway towns of Kent in south-east England (the Medway towns perhaps have the worst reputation of any in the county, and Chatham is definitely the least desirable of them). The term derives from the local criminals' tendency to smuggle cigarettes, lighters and drugs into the police cells, by secreting them in their anuses. Chatham criminals are so notoriously inept that the local police believe they know they will get caught, and consequently have to prepare for a stay in the cells by smuggling their home comforts in their "Chatham pocket".
I lifted that idiot lad again for swiping CDs from the market. No doubt he's brought everything he needs in his Chatham pocket.