You don't mention which variety of Magnolia you have - most (specially the evergreens) do not particularly like sun, so possibly you have it in a sunny spot, or its exposed to a lot of wind. Otherwise, watering may be the problem. New plants like these need a couple of gallons a week for their first 2 years during dry spells in summer.

I have Magnolia Stellata and find it loses all its leaves during Autumn New growth starts early Spring with a profusion of blossoms which die off and leave me with foliage for the rest of the Summer

Is it the outer (more exposed) leaves turning, Digitdeep, or the inner (older) ones?

hi, thanks to all who answered, i am not sure which type of magnolia it is, itisnt evergreen. it is in quite a windy position, buy is up against a high fence so i thought it would be protected a bit more, the outer leaves are crumbling and browning, all over the plant but there are healthy stems with no browning, i dont know if i should cut the brown ones off or just leave it. many thanks to all

Magnolias don't like wind, that's for sure! If you can't make a windbreak for it--build a temporary one with poles and sailcloth until a more permanent hedge grows, say--it should be transplanted into a more sheltered spot. Don't wait too long: established Magnolias also don't like root disturbance!