The drama, which last year starred Timothy Spall and Joanna Page as supermarket workers who won the lottery, was an instant ratings hit for the channel, attracting more than 6m viewers each week.
In the second series Siobhan Finneran, Alison Steadman, Mark Addy, Natalie Gavin and Jimi Mistry play a group of hospital staff who find their lives transformed overnight when their numbers come up.
Each episode focuses on the life of one syndicate member, revealing some dark and hidden truths about their relationships.
In the opening episode the group’s luck claims its first casualty as former syndicate member Helen (Sally Rogers) is forced to watch from the sidelines as the winners celebrate their good fortune.
Mellor told seenit.co.uk she hadn’t initially planned to write a second run but had been asked to do so by BBC Drama boss Ben Stephenson following the success of the first series.
The decision to set the story in a hospital allowed Mellor to explore the effect of winning on people committed to a vocation, a storyline she says wouldn’t have worked in last year’s supermarket setting.
“Ben came to me and asked if I’d like to do a second series, and I thought about it and there were some things I didn’t get said and I saw a platform to be able to say some bigger things,” Mellor told this site.
“Once I’d started to get those stories happening, I decided I’d do a second series.”
With episodes skilfully moving between comedy and dark drama, Mellor suggests fans of the first series were attracted to is “mixture” of tones and said her overriding ambition was that audiences “felt satisfied when they’ve watched an episode, that they feel like they’ve had a story and that they know more’s coming.”
The writer, who is currently working on a US version called Lucky 7, said she’d been approached “tentatively” to write a third BBC run “if the second series goes as well as the first”.
She also revealed that the third series, if it goes ahead, could be set in Sheffield and that she’s already thought about basing the story around a group of domestic cleaners.
The Syndicate returns to BBC One and BBC One HD later this month.