Find Ancestors

Hello All. I have come up against my second brick wall! Some of you on here helped me over my first with great success...I'm hoping you may be able to do the same. My Great Grandfather - Alfred George COOK b. 28/07/1892 at Aldershott Army Barracks. D. 30/07/1951 at St. Marys Hospital in Portsmouth. He was married twice - 1st to Emily Ada HICKS in Winchester on 17/04/1915 and then to my Great Grandmother Gertrude Ellen May DIXON in Funtington, Sussex on 21/06/1919. All seems normal? Well, my family name from my Mothers side is BORDER. I cannot for love nor maney find out why the name changed from COOK to BORDER. I have Alfreds parents down as William J COOK b.1861 in GLoucester and his Mother Mary Ann MILLENS b.1862 in Barnstaple, Devon. I also have these as his parents as this is what I was told by family, Father James Thomas BORDER b. 1851 Ilsington, Devon and Mother Lucy Horswill b. 1852 in Brent. When I spoke to the family about this confusion it was very hush hush and no-one is saying a word! I understand one of my 1st Cousins Frederick HARKIN (who descends from Alfreds first marriage to Emily Ada HICKS) he said he asked our joint GReat Grandmother, Gertrude and she said, I quote, 'It's something you don't need to know' - this goes back to the mid 1980's as Gertrude passed away and took the 'secret' to her grave. So! That's the story, can anybody help?? Yours in eager anticipation Nick