Another Salix Kilmarnock question arrived in my in-box last week: Trevor sent me this photo:
I don't know whereabouts Trevor and his willow are, but if you are anywhere in the UK then the simplest answer is the obvious one: it hasn't rained for weeks, and they need more water.
One of those leaves looks as though it might have had a leaf-miner in it (see Bugs, later) but another has a brown tip, so on balance I think the problem here is lack of water.
Willows are very water-dependent trees: they soak up a lot of it, and when grown in a pot (and I think I can just about see the edge of the pot) it is easy for them to dry out, especially when it hasn't rained for several weeks.
Even though it has not been particularly hot, it's been windy, and wind is very drying. After all, that's why we still hang our washing out on the line, even in the middle of winter! It's the wind that dries them, more than the general temperature.
To check, tip some water onto the surface of the pot. Does it sit on the top? Your pot is bone dry. Does it disappear immediately all round the edges of the pot, then run out on your feet? Your pot is bone dry, and the soil has shrunk away from the sides of the pot. Does it disappear down, apparently through the soil (rather than round the edges) but still run out on your feet? Your pot is bone dry.
In all these cases, I would suggest firstly giving it a good soak, then adding some liquid feed to the water and giving it another good soak, and finally, keep watering it every day for a week or more, even if it is raining, and with luck it will recover.
Willow are remarkably resilient, and dropping leaves is, for them, a very easy way to cope with the first few weeks of drought: as soon as the rain returns - which means, in this case, Trevor and his watering can - they will produce a new flush of leaves.
Oh, and about the Bugs: check out this post on another Sad Looking Salix Kilmarnock, for details and more information about watering and feeding.
Trevor, I hope this helps!