Unfortunately, pruning usually spoils the shape as many 'water-shoots' grow from the cut place. These are very straight and look odd. Pruning will also encourage it to grow stronger too. You may be able to remove a couple of large, lower branches to regain space underneath the tree. If so, rub out all but one of the sets of new shoots or it will look like a witches broom.

There is a good sized Magnolia next door to us. It does send shoots straight up if cut back too much. Our neighbour did raise the under part so he could more easily cut the grass. He stops it coming over our fence too. It is so lovely when it blooms. In this weather it gives welcome shade. It has been trimmed for many years by the previous occupants gardener.....but never drastically. The neighbour is going to borrow our extending pruner to try and deal with the 'water- shoots'. Most trees will give some sort of a problem in a smallish garden...... eventually.

Thankyou Volunteer and Doriac,
I'm not sure if I really want to prune my magnolia tree now, I don't want to spoil it, like you say they really are magnificent. but thankyou anyway...Poppyhans.