All on one half, one side, or confined to how many branches? or dotted about all over?

All one side, Bamboo, 2 branches with lots of leafy twigs The leaves don't appear to be drying out or changing colour, just curling inwards. One of the central branches of the tree has leaves just beginning to change colour. The rest of the tree looks normal and still green.The tree has been in 4 years.

Check the branches, all the way along, and that side of the trunk, to see if there's any damage of any sort, soft areas, or weeping, especially where the branches join the trunk. I don't know what it is, or if its a serious problem, but I don't like the sound of it! There's a list in the link below of problems they can get if its any help - I know you haven't said mottled leaves, but it lists other things as well

http://www.gardenadvice.co.uk/advisor/gardening-tips/mottled-and-discoloured-leaves-on-amelanchier-lamarckii/

No external sign of pests but how about internal such as a woodborer of some kind.

Thanks Bamboo and Loosestrife. I will give it a thorough check once the rain stops, and I'll look up the website mentioned.

could a neighbour have sprayed something in their garden and its drifted onto yours?

are they curling downwards/inwards or up/outwards?

The leaves are curling longitudinally inwards but it isn't caused by a spray as it is in the centre of my garden and any spray used by neighbours would have touched a lot of other things and would have had to travel quite a distance.
I have a nasty feeling, today,that the condition is spreading and some of the leaves are getting spotty. I will examine it more carefully tomorrow.