The mite Aceria cephalonea causes small red pouch galls on the upper surface of Sycamore leaves. When fullty developed in summer, the galls are less than 3mm high with a rounded apex. 

Aceria macrorhyncha on Sycamore and Aceria myriadeum on Field Maple

Gall Adult

Some indication of the size of the gall is needed.  The host plant must be specified in the comments box - e.g. "Galls on Sycamore, 5-6mm high".  In Spring and early Summer, before fully developed, these galls may be indistinguishable from Aceria macrorhyncha galls and should therefore be recorded as the species aggregate Aceria cephalonea agg. 

On Sycamore

From Spring to Autumn

Widespread and fairly frequent in Britain.

common

See Buckton, S.J (2023) Lessons from verifying Yorkshire galls and gall-causers on iRecord  Cecidology 37 (1) 28-40