What is a Home office reference number?

If you have a Home Office Reference number, you shouldn't have a passport... at least not a British one. You may have a Travel Document... in which case you can find your HO Ref in the observations on page 2. (If you do have a British Passport, your HO Ref is now effectively redundant as you've been made a British Citizen)

It should also be listed on any correspondance you've had from the HO, or your Status letter etc, where it says "Your Ref:"

Failing that, call the Immigration Enquiry Bureau on 0870 606 7766 and ask them :o)