How can I get rid of Honey fungus? It has killed one willow and appears to be attacking another - can it be saved?

Quck answer - No. Advice from various authorities say that it's best to remove the affected trees/shrubs and also any others close to the fungus, as they will be affected, as well.

Google the RHS advice on Honey fungus - it's very helpful.

Sorry you've got this...nasty. :-((

Honey fungus is a very interesting subject - its cropped up in a garden recently that i am involved with, and appears to be a particularly vicious form of it. As far as I can gather, its a bit like flu - some strains are not too nasty, and then there's the really nasty killer ones, although they're all honey fungus. All the advice I have read suggests that there's not much you can do other than
1. Remove any and all dead wood from shrubs, trees, etc., including roots if the tree is dead anyway.
2. Feed the remaining plants regularly during the growing season to try to maintain resistance.
3, Remove immediately any shrub or plant which has any evidence of honey fungus or die back as quickly as possible, including roots.
4. There's a list of plants which are more resistant to honey fungus, I believe it's on the RHS website - I remember it included Choisya, I have the list somewhere if you cannot find it.
Don't bother to remove the fruiting bodies (toadstools) it makes no difference - it's the "bootlaces" in the soil that cause the trouble, and they spread far and wide.

there are several species of honey fungus, sme move into aold dieing wood, others attack healthy stuff. I had a less aggresive one that affected older plants and i got fantastic advice from thr RHS as spritz has already suggested.