NI for invigilators

I understand that as an invigilator I do not pay Class 1 NI even though I correctly pay income tax via PAYE. Does this apply to all formal exams in the educational setting (ie including the ones the institution set themselves) or just externally marked papers?Thank you.

Hi Mum25 Trainee,
You may wish to refer to the National Insurance Manual here with regard to your query - NIM74100 - Class 2 National Insurance contributions: special cases: examiners and moderators
This confirms that the special arrangements for invigilators apply as long as the examination will lead to any certificate, diploma, degree or professional qualification.
If you are still unsure how this relates to your particular circumstances you will need to write to us at 
HM Revenue and Customs
National Insurance Contributions and Employer Office
BX9 1AN
Please provide as much information as possible in your letter.  
I am afraid this type of enquiry cannot be dealt with over the telephone.
Thank you.