Toni-
Have grandfather's birth certificate without middle name but all other records I've found has the middle name. Older sister said he added name after death of his father (aged about 15 or so).
Other grandfather- have record of name from family Bible (no birth certificate), school, marriage records, etc. My grandmother (his wife) told me why he changed the name.
My birth certificate records one middle name which would have been used on school and medical records, etc. After I went to court (not by Deed Poll as in U.K.) to change the name I had to apply for a new (amended) birth certificate and a notation was made with the change on the origional record. My passport and other records all have new middle name as it is my legal name but there were a few legal forms I've had to fill out that asked for all previous names.