Just to back others up:
Please tell your informant that his/her information is out of date.
At one time all yellow lines, both double and single, required a plate to display the times at which they operated. However, since January 2003, the requirement for a plate to be used in conjunction with double yellow lines no longer exists. (The only exception is when the parking restriction is only season, as at some seaside resorts, when the plate will then define the relevant months, rather than times of the day).
Chris