I had abandoned this EV, but after reading the interesting postings I have taken it up again.
I am almost there. I have got the theme but not what the 11 extra letters spell - though this is not important.
I have G* T* THE B*LL which must be Go to the Ball - very appropriate but is it a prize?
I have never seen it, but have heard of some of the thematic surnames. The family tell me that one of them is double-barrelled, which must be 12D, but I cant find the other part of the name.
I am also told that the missing one is Shirley something ( beginning with B) I dont want to look at Google yet, but as it is 6 letters it is probably Bassey. The letters are spread out in the grid though which seems unusual.