There is a lot on the internet if you type in 'weeping willow + diseases' Crown gall and root rot seem to be two quite common troubles. A friend has a huge one in his garden, which he has recently had the underside lifted higher. I always worry it is in striking distance of his house. They are beautiful trees but get so large.

It could be bacterial anthracnose. Are there small blackish lesions on the shoots?

Thank you for your help- There are brown flecks on the top and underside of the more yellow leaves.

These dark brown almost redish marks could be called lesions and are on some of the shoots.

It does sound like bacterial anthracnose.

pdf file here about it:

http://www.bartlett.com/resources/Anthracnose-of-Willow_UK.pdf

ps, it's a fungus too, not a bacterium.

Yes, it does sound like that, not sure that there is too much I can do then? Thank you Georgie