This wouldn't have anything to do with exposure to a 1964 movie starring Don Knotts called "The Incredible Mr. Limpet", would it? Many people who are exposed to the movies and/or television performances of Mr' Knotts develop an unhealthy fixation known as "Knottsmania". It is a relatively common disease here in the States and it just may have jumped the Pond.

Here are some common symptoms:

1) Phoebe walks around asking "Where's my bullet?"
2) Phoebe begins referring to Liz as "Aunt Bea"
3) Phoebe begins referring to Manitas as "Andy"
4) Phoebe refers to any of her friends as "Opie"
5) Phoebe begins driving a 1962 Ford police cruiser with "Town of Mayberry" on its door
6) The Dillards (as recorded in the early 60's) becomes Phoebe's favorite band

If Phoebe exhibits any of these symptoms, just be patient. There is no known cure for Knottsmania, but it is usually a relatively short-lived illness. She should be back to normal by the time she reaches puberty.