If it was just on the grass I'd have said Snow Mould but never seen that on a tree trunk. Try googling it and see what you think.

Thanks for your comment. I have googled snow mould but it doesn't look the same. Snow mould looks more like a brown patch on the grass. It's hard to tell what this may be, the white around the tree has dried and become powdery but the new patch on the lawn is soft and dark inside. My neighbour who is a trained gardener said he hasn't seen the like before!!

Soi all I can think of is to ask the British Mycological Society in Manchester the phone no of your nearest branch and ask them. Or ask if they could identify from a photo.

Great will do that thanks! Linda

It's a slime mould - not terribly attractive, but harmless. You might be able to disperse it by blasting it with a hose.

Wow, never realised they got as big as that!

That's great thanks! I'll have a go at hosing it down, although it's turned grey now so looks like it's going...