The fungus is a pathogen of dandelions and causes the leaves and flowers to distort and can cause rapid die-back of the plant concerned. It is common and widespread across the UK.
The fungus had dramatically distorted the leaf, causing it to curl over. 

Here you can see the orange, blister like sori, on the underside of the leaf, that contains the sporangia.

View of the upperside of the leaf.

The round angular sporangia release zoospores at maturity, viewed microscopically at home.