Don't usually prune at all, but remove anything that's dead back to live wood - sounds like the top has been killed back by something.

Mr Lloyd says in his 'Cuttings' book, which I am reading at the moment, March for winter flowering Mahonia. I leave mine to make those enticing berries which starlings and blackbirds like. Then , if I feel like it, I get out the long pruner out and reduce height. Just try not to let them get too scrawny and bare limbed. If it is another type of Mahonia I don't know. As the lower down type get used for supermarket planting, and the contractors come regularly, and scalp every thing, I expect they can put up with anything.

many thanks for your help