I am not certain that there is an inability to charge, thats the whole point to it.
An examiner who is a PPL holder may not be remunerated for any flying that relates to the privileges of a licence maintained in accordance with JAR-FCL. JAR-FCL 1.110a.
National PPL holders may still be remunerated for instruction and tests conducted on Microlights, Autogyros and SLMGs.
NPPL holders cannot at present hold a FI qualification and therefore can't hold an examiner authority.