I have planned a purple magnolia earlier this year.

Pick off any snails you can see all over the plant, spread slug pellets around the base of the plant and in the surrounding area to deal with them.
As far as the brown patches are concerned, where exactly are they on the leaves? Starting from the edges and working in or in the middle with green round the outside? Any evidence of damage by snails, etc., in the form of holes or ragged areas near the brown patches? Also would like clarification of what plant you've actually got - purple magnolia's a bit too loose! Is it evergreen?

Its a Magnolia Susan and I am presuming it is a hardy plant as the instruction is to plant in sun/half shade. Brown patches start at leaf tips extending inwards. Seems to be independent of snail damage. Do you think it is just dying? Thanks for any help you can give me.

This sounds more like a drought problem - don't know how big the plant is, but we've not had enough rain to soak its roots often enough - it would have needed a minimum of 5 litres a week, even twice a week, in its first year. Will check the pest info I have though and get back to you if there's something else that's likely.

Thanks for that, it is just under a metre high. I will increase the watering and keep my fingers crossed. Will let you know.