They’re rubbish, aren’t they?! You need the IN 2016/003
Civil Aviation Authority INFORMATION NOTICE Number: IN–2016/003 Issued: 8 January 2016 Procedures for Examiners Holding Part-FCL Certificates Issued by Countries Other than the UK
It’s a dull read but they are really picky about the format of the title of the email you send requesting authority.
Good luck