Joining the cast for the one-off special are Tim Preston as Gary and Phoebe's son Michael Sparrow, a returning character from the original series now a teenager; and Esme Coy as Yvonne Sparrow's 21st century daughter, Ellie.
Esme is also Emma Amos's real-life daughter and they are appearing on screen together for the first time.
The cast were pictured today in central London at the first read-through of the script for the new episode, although Victor McGuire was unable to attend.
Producers Retort explain: "It has been 17 years since Nicholas Lyndhurst played the character of Gary Sparrow, a former TV repairman and accidental time traveller who leads an extraordinary double life. Now the original creators Laurence Marks and Maurice Gran (Birds Of A Feather, The New Statesman) have written a brand new script that sees Gary Sparrow face the consequences of his time travel and go somewhere he's never been before, as Goodnight Sweetheart is about to be catapulted into the 21st Century."
Announced last month, the special episode is set to be broadcast during the BBC's Landmark Sitcom Season later this year. Plans for the season are currently being finalised, but the programme could be on air as early as September. Other sitcoms being revived include Porridge and Are You Being Served?, although Goodnight Sweetheart is the only show to be both written by and starring its original cast.