Cheers for the comments. The wilting branch made a recovery yesterday- perked right back up. But today its really wilted again. Rest of plant is fine still tho as are other plants. The soil moisture is fine I think- I don't leave it saturated all the time- I prefer to soak it till its very wet then let it dry right out and water before they wilt.
Humidity- well there's a big problem. UK summers are always humid- this year even more so. Been mostly damp with temps around 70F. Hotting up to 85+F this weekend tho! Very very humid though.