hydrangea bushes there are best pruned in the spring just as the buds start to break but if you need to prune them in the autumn or winter try to prune back to and active node on the stem, this is the point were the leaves join the stem or did join the stem in which case its a ring with some dormant buds on it. You should be able to prune up to two thirds of the total stem length.
Then over the winter I would protect the plants with some horticultural fleece just to keep the worst of the frost off the plants.
In the spring because of the hard pruning expect to loss a few of the stems, however if you feed the plant with a general plant food such as growmore it should recover by sending a number of new stems out from the base of the plant.