This gall is very common on nettles, and can appear on the stem, leaf stalk, leaf, or the (very small and inconspicuous) flowers of the plant. The gall is caused by a fungus, Puccinia urticata ("nettle rust", or "nettle clustercup rust").
Aside from causing obvious swelling, the gall can be seen to be dotted with numerous so-called cluster-cups, which have orange discs and a paler yellow margin; these cups are called "aecia" ("aecium", singular).
The rust fungi, of which this fungus is one, have an extremely complex life-cycle. Some of them can produce up to five different kinds of spore; the different spore-producing structures are designated, in the scientific literature, 0 (zero), I, II, III, and IV. Species that produce all five kinds of spore are said to be macrocyclic; those that do not are microcyclic. Puccinia urticata is a macrocyclic rust; two of its five stages are hosted on nettle, while the other three are hosted on a species of sedge.
As "British Plant Galls" (Redfern, Shirley & Bloxham; 2002) states, "it seems likely that, amongst all living things, the rust fungi have the most complex life cycles and nuclear arrangements".
The cluster-cups shown here produce asexually-formed "aeciospores" (stage I of the life-cycle); inspection through a hand-lens will often show that some of these discs have a yellow powdery mass adhering to them, made up of spores that have emerged from the discs.
This specimen was on a plant growing beside the path shown in NS3679 : Access to Asker Farm and covered reservoir