im not any sort of an expert but i would be afraid to cut it back now cos that would expose the cut ends to the ferocious weather? maybe in a couple of months? i dont know where you are gardening ~ that may make a difference ~ depending on how severe the winter is?
welcome to GoY by the way!

Hi Ronadare and welcome to GoY. If you cut back now you will remove this years flowering buds. It isn't usual to prune a magnolia at all you would normally only remove any dead wood between November and February.

IF YOU HAVE TO - prune in April May, after flowering. That'll give it time to make up some good growth in time for next spring. The harder you prune, the more new gowth will be made, and, perhaps oddly, the less ripened flowering wood will be produced, because the new growth will be vigorous, and immature. Lighter pruning will probably produce more flowering wood, but it'll make the growth congested, so I'd prune sensibly, not nibbling at the edges, and expect a slightly reduced flower show next spring. Start by removing the smaller crossing branches, and anything that clutters up the centre of the tree. The aim is to keep a good air flow round all the growth. As MG says, pruning isn't normally necessary. Worthy

Grateful for the advice. Fortunate with the weather here in Poole and less than a mile from the coast so climate is fairly mild, although had snow for several days like most. I shall leave it a few months before picking up the secateurs and do some 'sensible pruning'. In ten years I have only taken a little off the top to clear the telephone cable (In a bungalow) but need now to reduce the girth.
Thanks for the welcome to GoY. I think my recent present to myself of the computer will prove invaluable!

i think so too ~ i would hate to be without mine!