Changing daughters surname

Fiona, your saying that I need to tell her now about her true parentage.
Shes 15 months old. Whatever I say to her she just laughs and claps her hands. I dont think shes going to understand.
She will be told as she grows up, that he isn't her dad, and I try and encourage my ex to see her.

It's possible to begin the process of developing an understanding of an unknown parent through play and art materials and learning through the use of photographs (looking at physical similarities between family members) and timelines etc.

As I say the evidence is that children who grow up understanding their parentage are more secure longer term than those who have their confidence shattered by learning a parent isn't a biological parent when they are older.

That doesn't make any sense. At 15 months old, she is more interested in eating a photo than looking at it. She is still too young to understand.
My health visitor and my solicitor have suggested introducing the concept of different parents at the age of 5 or 6, and said it may not stick even then.
I know of a child who is 6, who has a different dad to her 2 brothers. She still doesn't understand that all though they have different dads, they all have the same mum. It's not something a child understands untill they are 8 or 9 at least.

I don't know what a child understands, however from my experience, I met my husband when my daughter was one year old and never called him dad to her even though he was a dad to her for more than 13 years. She chose to start calling him "dad" when she was about 7 or 8 years old and started having friends around for tea. She has a brother 4 years younger than her and my stbx is his dad. She has never had very regular contact with her biological father, however I would not have wanted to start explaining to her which one was her dad even at 5 years old. Somehow the truth always seems better to me.

I, too, wanted what I consider a "normal" family life for my daughter. Sadly this has not been the case and two dads have let my daughter down.