Forbes later said "I got the reputation as a fast man with a pen, and as a result of that I got my first real screenplay assignment for "Cockleshell Heroes"." José Ferrer had Bryan Forbes's script rewritten by Richard Maibaum, who had worked on several Warwick movies, which Forbes admitted "I didn't appreciate." Filming started in March 1955. Filming was done in Portugal and several Royal Marine establishments, with the Commandant-General Royal Marines training the actors for drill and canoe handling. The training camp scenes in the film were shot at Eastney Barracks in Southsea, Hampshire, now the home of the Royal Marines Museum. Many of the barrack buildings seen in the film still exist including the military buildings further up the beach where the scene to dispose of the live explosive devise before its fuse time expired was filmed. The Royal Navy ships, and , were used to portray a German anti-submarine vessel dropping depth charges. Studio scenes were shot at Shepperton. The limpet mine scenes were filmed in the King George V Docks in North Woolwich and many of the other scenes were filmed on the adjacent bomb sites and at derelict houses in the area. Lieutenant Colonel Herbert ""Blondie"" Hasler, RM, the leader of the real-life raid, was seconded to Warwick Films as technical advisor. Ex-Corporal Bill Sparks, the other survivor of the raid, was also an advisor. "It was not a happy picture at all," said Forbes. "Ferrer was a megalomaniac." The film briefly uses several railway locations including the level crossing (Military Road) adjacent to Fort Brockhurst railway station on the (by then 'goods only') Gosport branch in Hampshire; the station buildings and former platforms survive today as a private residence. As he cycles south, José Ferrer has to wait for a passing northbound train (a van hauled by T9 class locomotive 30729) so he takes the opportunity to abandon his bicycle in favour of a ride in the rear of a fish lorry. Later Ferrer steals the fish lorry only to abandon it at Shepperton Station (Surrey) in order to catch a just-departing Up train allegedly to Portsmouth, steam train noises being provided on this otherwise electric branch. In another sequence David Lodge ducks out of sight into a brick bus shelter alongside the North Woolwich Branch. This was possibly at the footbridge opposite Fernhill Street on Albert Road, west of North Woolwich station.