The Smile hitmaker took a career break in 2010 to concentrate on starting a family with her now-husband Sam Cooper, but she announced last year (12) that she was eyeing an end to her temporary retirement.
As part of her comeback, Allen contributed to a cover version of Dream a Little Dream on a new album by former Take That star Williams, and she spent just half an hour recording it.
She tells Britain's Hello! magazine, "Robbie's a really sweet guy. Really humble. I just worked with him. It was really quick, actually - we just met up in the studio and then left. I think it took us half an hour."