Question
how and when to prune this is a mature tree that is quite tall and full. any advice?

Answer
The best times to prune willows trees are in the late summer or early fall. This variety bleeds sap, so it is important to prune when the sap is bleeding the least. I would think more of "giving it a hair cut" than the normal pruning of cutting off branches. The umbrella shape will not hold all year --the willow branches will grow back to its weeping look as the summer progresses so this "hair cutting will be an annual task. Remove the dead branches from the tree but try to limit the cutting of many live branches.