As the DUAL flight is not a test! the instructor has no authority to impose any particular standard. He can give advice if the standard is lower than he would like, he can refuse to rent an aeroplane to the candidate, he can refuse to sign if he so wishes but, if the candidate has received 1 hour of instruction from an FI then he can legally claim to have met this JAA requirement for revalidation. If his rating is not revalidated purely on the lack of a signature when the requirement has been met, he has the right of appeal under CAA Regulation 6.