The pair, who split up last month, were spotted together at a West London hotel earlier this week.
Zane apparently persuaded the model to give their relationship a second chance.
According to The Sun, the Titanic actor flew to London from a film set in South Africa and bought chocolates and roses for Brook.
"Kelly was giggling and looked very happy," said a source. "She kept touching his arm."
A witness added: "Billy looked like the cat that got the cream. But she seemed to be teasing - it looked like she was keeping him on his toes."