“Thicko” Derwent–Broxley is a non-canonical character, created for the 2022 miniseries Why Didn't They Ask Evans?, an adaptation of the Agatha Christie novel of the same name. He is Frankie's cousin, belonging to the Norfolk branch of the Derwent family.
Spoiler warning: A spoiler is announced! The following section contains details about the plot of Why Didn't They Ask Evans?.
After Alex Pritchard dies, Thicko is sent to attend the inquest in Frankie's stead. He hears the testimonies of the Caymans and chats with Bobby for a bit, explaining how exactly he is related to Frankie.
In the last episode, he is present at Frankie and Bobby's wedding.
Spoilers end here.