Half of the shrub (one of the two main branches) immediately responded quite happily and sent out new shoots but the other died. I thought I might have done some root damage at the time digging out all the cooch grass around it.

Now the other half has dropped off its twig, so to speak. This is 4-6 weeks after the "incident". It seemed to be bushing out quite healthily, but then the leaves yellowed and are now dead brown - over just a few days. Any ideas please? So I don't do it again. It was quite a severe prune, but I did leave what I thought was sufficient leaf coverage that it could make food, and since it seemed to come back OK I thought I hadn't overdone it.