I live in Derbyshire, UK and have a Magnolia tree which in recent years seems to have gone a bit out of control, i.e. much bigger than I think it ought to be for the space it occupies.
I feel it needs to be half to two-thirds its current size, but am unsure if such brutal pruning would harm it. Also, when is the best time of year to do so? I have included a picture of how it currently looks.