I believe they do not legislate to that level of detail, but in the British system, first of all, it is not the judge who decides on the appointment of the special advocate, contrary to what is proposed here; it is the government. The government has discretion to appoint or not to appoint a given individual, and the information we have is that in the way the program is administered, the government takes into consideration that risk of tainting.
On December 7th, 2007. See this statement in context.