He joined the Royal Shakespeare Company in 1963 in A Midsummer Night's Dream and appeared as the young boy (during Ringo's walk) in The Beatles film A Hard Day's Night (billed as "David Jaxon").
He also replaced Richard Gibson as the Gestapo officer Herr Otto Flick in the BBC sitcom 'Allo 'Allo!
He has also appeared in a variety of other roles including in Softly, Softly, Doomwatch, Jason King, Dixon of Dock Green, Z-Cars, Brush Strokes, T-Bag Strikes Again, Ever Decreasing Circles, the postman in Keeping Up Appearances, The Upper Hand and Don't Rock the Boat.
[1] Janson was married to actress Debbie Arnold, who played April Branning in the BBC One soap opera EastEnders in 1996.
The couple worked on the Streamline English as a Foreign Language video series.