I have to say, this is the same with me. I don't have a valid passport and they have asked for my birth certificate reference number with the declaration form. I'm tempted to not put a number down in case it's wrong and send the certificate in a hope that they can work it out themselves.
I have had my form returned saying it is incorrectly completed. I have the small birth certificate and i put the top letters and numbers on the form. My mother has two small birth certificates and its the bottom numbers and letters that are the same on both certificates. I have sent it of again with the bottom one on, lets hope this is right.