The undersides of the leaves are covered in an orange rust, with fuzzy black patches.
Zoom in and those black patches look like this, kinda bushy!
Those black 'bushes' are clumps of teliospores that look like this:
|Teliospore mounted in water. 100x magnification.|
I reckon what I've got here is Phragmidium tuberculatum. The narrow pointy bit on the end (the 'apical papilla') is a distinctive feature of this species, according to Ellis & Ellis's Microfungi on Land Plants.
For the record
Location: Woods Mill, Henfield
Grid reference: TQ218138
Record entered into FRDBI 07/09/2018