Joel Britten is a non-canonical character created for ITV's 2007 adaptation of the novel At Bertram's Hotel. He and Jack, his twin brother checked into Bertram's Hotel at around the same time as Miss Marple near the beginning of the show.
Jack and Joel looked very much alike and both were in fact portrayed by the same actor, Nicholas Burns. The brothers used their strong resemblance to each other to further their purpose which was to steal jewels from guests of the hotel. Joel first appeared in the hotel room and then reappeared as his brother to create an alibi for him. See the article for Jack Britten for details on their modus operandi and how they were exposed.
While in the dining room, Joel also helped Inspector Larry Bird break down the door to room 123 where a sniper was shooting at people in the street.