JAR-FCL 1.080 (c) (iii)
The holder of an instructor rating may log as PIC all flight time during which he acts as an instructor
(iv) The holder of an examiner's authorisation may log as PIC all flight time during which he occupies a pilot's seat and acts as an examiner.
Foxmoth
As an Instructor you should be familiar with JAR-FCL, it contains most of the answers!