I use this
It comes in a slightly different bottle in Wilkinson's and is about £1.89.
I spray the children with it before school, parties etc and in 5 years of school/pre-school and 3 children, we have not had head lice. Yet.
I know it's probably tempting fate to say that, and I expect we will eventually succumb, but I'm sure that the spray ha helped a lot.
I also combe through with conditioner and a nit comb every time we hairwash. Also, I tie up the girls' long hair too.