This was a programme broadcast on 28 May 1966 on German television, and later in Austria and Finland. Victor Canning is credited as the original author and Oliver Storz for the translation and adaptation.
It has recently (March 2014) been issued in Germany on DVD.
The title translates as "the really big thing", or more colloquially "the big idea", referring to Marty Fowler's notion of pulling off a major criminal coup with this substitution. The synopsis does not match any published novel or story by Canning, so it is probably based on one of Canning’s missing TV plays, perhaps Double Stakes from 1963 or Come into my Parlour from 1965. It may also be a play that was never broadcast in the original English version. There is no similarity to Canning's novel The Great Affair.
The German website Die Krimihomepage gives the following synopsis (my translation):
Dickie Gray has spent several years behind bars for forging cheques. When he is released, he promises his wife Carla that he will lead an honest life. Carla joyfully tells him that his previous boss is willing to give him back his old job in a travel agency.
A few weeks later Dickie runs into his erstwhile accomplice, Marty Fowler, in a London bar. Marty realises that Dickie bears an uncanny resemblance to Gerald Bennett, boss of a famous jewellery store, and sees this as a chance to commit the perfect crime: kidnap Bennett and install Dickie Gray in his place. Marty’s mother Flora is brought into the plan along with her childhood sweetheart Jimmy Warren. Warren is a cold-blooded gangster and head of a syndicate. If the idea succeeds, then he will take over. Dickie Gray wants nothing to do with the idea and declines.
Four weeks later, however, he is sacked and can’t find another job, so returns to take up the offer. The crime is meticulously planned, with Bennett’s daily routine carefully recorded on film and his phone calls monitored. Dickie Gray, once a struggling actor, studies the character and habits of his double down to the last detail. When the big day comes, nobody in the jewellery store notices that it is Dickie Gray rather than Gerald Bennett in the director’s office. The plan goes smoothly until Dickie, who abhors violence, learns that Jimmy Warren wants to do away with the real Bennett.
Now the plan begins to unravel.
A minor error in the filming was that Dickie Gray's release from jail is filmed in front of Holloway Prison. German viewers were obviously not expected to know that Holloway was at that time exclusively a women's prison.