The larva of the midge Dasineura tympani causes a blister gall on Field Maple (Acer campestre).   Galls are 4-8mm across, and turn brown once the larva has exited via a small hole underneath the leaf.

Acericecis campestre galls are similar, but the white larva is in a depression under the leaf

host

Photograph upper and lower leaf surfaces, and ensure you have noted the host plant species in your comments. 

Field Maples - woodlands, hedges, etc.

rarely recorded