This is what they look like on the third day. Definite mushrooms today, and slightly lighter in colour now.

Having searched and searched on Google, I think I’ve solved the mystery.  I found this photo with these details:
Hypholoma fasciculare Sulphur Tuft

Orange-brown with sulphur-yellow.
May – on stumps of deciduous trees.
I know it’s October but the weather [...]

It happened overnight.  One day nothing and the next day we had been invaded by a army of tiny, bright orange mushrooms!  There was a small platoon a few feet away, hiding in the rose bush . They were nestled down in the roots, but Mo’s keen eye found them.
What are they after?  Will they [...]