I don't know for sure but the very hot weather seems quite a likely culprit, maybe the leaves are actually sunburned. They will not only be getting the hot sun beating down on them but reflected back up by the patio. Any chance of moving them into a shadier position for a while?

Try feeling down a little way into the soil and see how dry it is a few inches down as often the water we put in that we think is enough only descends a little way down and it is surprizing how dry something can be. During the colder months the soil seems to keep it's moisture much better. Also willows do need to drink quite a lot. I have a wisteria in a pot on my patio and that was doing fine until a week or so ago and that now looks like it's leaves are getting dry. When I watered it last night I also watered all the leaves as well and hope that it will be okay.
I would try giving them a good soak and see if that works. What kind of tubs are they in? If they are a plastic type then they will stay moist longer than a terracotta pot. They are obviously good specimens at that height so will need a lot of water. Maybe 7ltrs between them is just not enough. Good Luck with them I hope they survive this heat.
Alison

Here is a link that might be useful: Alisons pond and garden

Thanks for your replies. They are potted in old wooden whisky barrells (bought from a garden centre - I don't distill my own!), that have been cut to about 1.5ft high. Unfortunately, with all the soil in them, they are very heavy to move. I will see if the soil is most further down.