A Oram

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?