She does have a small crest on the head behind the comb (I don't think you can tell from the photos), much much smaller than a normal legbars.
I've just looked at some photos of the cuckoo leghorn and there does seem to be some resemblance, but I really don't think it's a cuckoo leghorn cross, becasue I've never come across a cuckoo leghorn even up for sale here in the UK, and i've just looked again at a popular UK site where i can get chickens from and there is absoloutley no cuckoo leghorns available anywhere. I don't even know if they exist in the UK. She gives me slightly less eggs than my Light Sussex, maybe one less per week, and the eggs are a little smaller than the light sussex's also.
Some more clue's from her personality: she's not as docile as the light sussex either, but she is not exactly flighty either. When I got five new chicks about 10 weeks old, she was the big bully. Seems to bully the chicks every chance she gets, although she has calmed down a little now after 2 weeks.
I think she is almost certainly half legbar, but I don't think it will really be possible to be sure of the half, will it?