I'm guessing this is some sort of virus or fungus causing the problem. The leaves appear healthy i.e. no marks or powdery appearance, before they wilt. Initially it was only one branch but now it appears to have spread to others on the same bush, albeit some branches still look very healthy. The fruit appears to be ripening OK. The bush did have some sawfly earlier in the season but this was dealt with by by a mixture of squeezing the caterpillars and lightly spaying with Bugclear (approved for fruit bushes)
What is the best course of action?
1. Will the affected branches recover (there is some hint of tip growth being less affected)?
2. Is there a treatment?
3. Should I cut out the affected branches.
4. Should I grub out the whole bush in case it spreads to the other three very healthy ones?