So far I told the HV 'I don't know' and she kept in blank. I know that ethnicity is considered to be a choice which is why it is now used in place of race, but I can't ask my baby what he chooses to be!
Basically, DS is 3/4 white 1/4 Chinese by genetics (I'm white). In fact his Grandad is from Indonesian and has never been to China, so whilst DH is very proud of his heritage (and wants DS to be too) he has always found it confusing to answer the inevitable race questions (he looks fully Chinese).
DS has a Chinese middle name that has been proudly chosen by his Grandfather. Er, anyone with children who are 3/4 White 1/4 Something Else - have you ticked 'Mixed Race' or 'White'?