Asked to pay Class 3 voluntary contributions when colleagues in same situation pay Class 2

I work abroad for an international organisation. I worked in the UK immediately before moving abroad 28 years ago (for HMG in fact) and have been continuously employed abroad since then. I applied to pay voluntary contributions via CF83 and provided proof of my employment record abroad as of the date I stopped working for HMG. I have been asked to pay Class 3 contributions and I don't understand why because: (a) for the tax years 2004/5 to 2009/10 I was asked to pay Class 2 contributions and I have not changed employer since then, and (b) I have British colleagues who pay Class 2 contributions although they are in exactly the same situation (worked in the UK immediately before leaving and are working abroad). How can I ask for the decision made in my case (Class 3) to be reviewed/corrected?

Hi A Oram,
If you are employed or self-employed overseas and were normally employed or self-employed in the UK before your departure then you may be able to pay at the voluntary class 2 National Insurance rate.  As you have advised that our International Team have accepted your application at the voluntary class 3 rate then may I ask that you write in and ask them to review their decision.  You could reconfirm your employment position in your letter.  
Our postal address is:-
HM Revenue & Customs
PT Operations North East England
BX9 1AN
United Kingdom
Thank you.