The first thing I would do is unroll the leaves and look for aphids on the underside. I would probably check their general culture, too. Rolled leaves can be a symptom of watering problems--too much or too little--or some nutrient deficiencies. Weed and feed can also cause similar problems, if applied too close to the tree.

Thanks Tugbrethil. I don't think its a watering problem as we live in a part of Wales that gets its share of rain but the ground is not waterlogged. I did look for aphids before I posted the question but didn't see any - but I had a similar problem last year on a nearby damson tree so it could well be an insect of some sort. Anyway I have mulched both this spring with rotted manure - perhaps I should spray both with a systemic ?- shouldn't hurt the damsons as the flowers have only just finished.

I have my doubts as to spraying an edible plant, or one attractive to wildlife, with systemics. I would spray the Magnolia, but avoid the Damson, at least if possible!
Rotted manure is good--it's the chemical weed killer in weed and feed that damages the tree. It acts like a not-quite-right plant hormone, distorting the new growth. I mentioned the possibility in case it was in or near a lawn.

The tree is in an island bed in a lawn, but a single glance would convince anybody that weed and feed has not so far been near it! Our morning chore at the moment is collecting dandelion flowers before they seed. Takes a while as its a big lawn. I have been planning to weed it though, but with granules, not spray. I don't suppose granules several feet from the tree would hurt it? It ought to be done soon to avoid the whole area being taken over by celandines!

Don't use granules--they are too concentrated, and will soak into the roots. Spray the weeds lightly with a liquid weed killer, just enough to wet the leaves of the weeds, without soaking into the soil. A surfactant mixed with the weed killer can help with this. Of course, do this on a windless day! Remember that the roots of the Magnolia--or the Damson--can be up to 3 meters away from the edge of the tree canopy.

Not as easy as it sounds - windless days here are few and far between - we did have one today - but all I had were granules. Ho hum.

Drat! Well, if you can use the granules far away from any trees, then they at least won't go completely to waste. If your spot is like where I went to college, where the wind never stops blowing, then try using a piece of cardboard as a windbreak as you spray. The main idea is to keep droplets from landing on the surrounding landscaping.