The Sycamore Red Pustules Gall is a gall caused by the mite Aceria macrorhynchus.
This gall occurs over most of Britain, and there are usually plenty on Sycamore leaves. The small galls poke up from the leaves as small red dots, and are hard to the touch.
Sycamore Red Pustules - http://www.flickr.com/photos/qwertyqwertyqwerty/