Assistant editors are responsible for maintaining a smooth flow of material from the production and post-production teams to the editor. Perhaps their most important duty is in organizing recorded material as it comes in so that it can be easily accessed by the editor. In the course of their duties, they act as the editor's key liaison to other departments. They generally do not directly assist in the actual assembly of the rough or final cuts of episodes.
They are usually required from the first day of principal photography to the last day of post-production. Editors, by contrast, begin their work only after sufficient material has been filmed and organized, and may not fully begin work until after principal photography has wrapped.
On most projects, they are usually brought onto a project by the editor, rather than hired directly by the producer or director. However, BBC Wales seems to have a regular editorial staff who rotate duties on various episodes. It seems likely that various editing/assistant-editing teams are assigned to particular production blocks by the prudction manager. Due to the overwhelming workload on even a single production block, various editing teams are used throughout the course of a season.
Career advancement can be detected in BBC credit rolls. Ceres Doyle, for instance, began as an assistant editor on Rose, but at least by Utopia had become a full editor.
This position has only been credited on-screen since the advent of the 2005 series of Doctor Who There may have been people who performed a similar functionality in the later years of the 1963 version of Doctor Who— indeed Tom Baker's wife, Sue Jerrard, was apparently an assistant editor on the original version of the programme — but the only editing credit given then was for film editors.