Mature ones should not be pruned as it can damage them.
There is an instructional video on line for trimming and shaping young ones. Its important to consider the size of any tree at maturity before planting it.

My personal view is 'never'.

The problem is it's shot up past the hight of the house so want to bring it back level, and there's a branch about a 6 foot up from the ground that's growing straight past a set of patio doors?

Which magnolia do you have as many of them are very large trees. If it is grandiflora (the evergreen one) for example that can grow to over 20 metres in height. Hard pruning is likely to cause the tree to send out 2 metre long straight shoots.

You can prune Magnolia, and I would if it is taking over my garden: it is best to do so in spring or summer. I would only prune out the most offending branches at first and then filter out other branches in successive years to bring it down to a manageable size. You can prune off the new growth now (the stem should be green)

I'm in the 'prune-it-when-it-comes-through-your-window' school. Hard on the lines of 'well, why did you plant it so close to the window?'

Gosh wish there was a like box on GoY Urbanite!

That was really what I meant as well, but its a bit late now!

No Stera never too late!

Thanks for replying some very helpful info