Thanks Matt, and yes I am indeed a full blooded Brit, shoulda made that clear I guess.
Having done a bit of research, it seems that the three years actually refers to the length of time you have been in the UK, rather than how long you've been married for, as per the rules from the newcomer link: http://www.ind.homeoffice.gov.uk/ind/en/home/applying/british_nationality/general_requirements.html
[nonactive] :
You must "Have lived legally in the UK for a minimum of five years before you apply (three years if you are applying on the basis of marriage to a British citizen) and are free from immigration time restrictions."
DH has the book on UK facts and figures so, fingers crossed, he'll be fine for the "exam"
Has anyone been in a similar situation? If so I'd love to hear what proof of residence in the UK prior to the marriage they sent in with the application...