Q:

When is the best time to prune a Magnolia tree, and how much? 

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. 

A:

You can do some pruning now, its not quite too late as its early winter.  However, heavy pruning all in one go is not advisable - deciduous magnolias can take a long time to recover from pruning, and hard pruning is likely to cause a lot of watershoots. 
If you want to reduce its size, the best way is to do it over 3 years, reducing the top branches and twigs by no more than a third, and preferably only by a quarter each time, but even then, you may find watershoots increase and the tree may not flower very well for quite a while.  Further info under Pruning and Training here https://www.rhs.org.uk/plants/popular/magnolia/growing-guide

