A source said: "Johnny was instantly attracted to Natalie. He told her he was a huge fan and suggested they meet to discuss working together in the future.
"He gave her his phone number and whispered that he would love to see her again."
Since the party on Valentine's Day, Johnny and Natalie have spent hours talking on the phone.
The source added: "There is definitely chemistry between them."
Natalie, 33, has been single since splitting from husband Daniel Johns last year.
Johnny, 27, was recently dumped by Hollywood actress Kirsten after she got fed up with his untidiness at the London home they shared.