I'd cut it now...its not going to get and better and for most of the blights where the treatment is cutting out, the sooner the better. https://www.rhs.org.uk/advice/profile?PID=96
Ty woof. Ive trimmed the worse affected one down hard. The other box hedge (diff part of garden) im undecided if it is blight or if it simply needs a good feeding. For those ive ordered some fertiliser and will spray with fungicide just in case.