DDs “dad” has had no contact or paid any CSA for two years. She is 4.
Around a year ago I text him and asked him to give permission to change DDs surname. NOT to get rid of his name but to double barrel it with my name. The only reason I didn’t do this when she was born was that I was still using my ex husbands name and didn’t want her known as that. I’ve changed my name now and would like it double barrelled for DD.
She’s young enough that it won’t make a difference to her but she’s starting school in Sept and I’d like it changed by then.
I text ex about a year ago and he replied (a miracle in itself). Categorically NO, I do NOT give permission 
I can’t legally change it without his permission. Could I get around it by using a “known by” name when she goes to school? So her certificates etc would be her birth name but she would be known by her new name?